diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -5,6 +5,15 @@
 ## Unreleased changes
 
 
+## Version 0.5.1.0
+
+-   **elynx-tree:** new functions `isValidPath`, `isLeaf`, `depth`; add conversion
+    topology -> tree; various internal algorithmic improvements; improved error
+    messages; simplified interface to Newick parsers; parallel fold map; Nix
+    flake.
+-   Remove unneeded dependencies.
+
+
 ## Version 0.5.0.2
 
 -   Speed up mixture model simulation.
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 
 # The ELynx Suite
 
-Version: 0.5.0.2.
+Version: 0.5.1.0.
 Reproducible evolution made easy.
 
 <p align="center"><img src="https://travis-ci.org/dschrempf/elynx.svg?branch=master"/></p>
@@ -71,11 +71,11 @@
 
 Handle evolutionary sequences.
 
-    slynx --help | head -n -16
+    stack exec slynx -- --help | head -n -16
 
-    ELynx Suite version 0.5.0.2.
+    ELynx Suite version 0.5.1.0.
     Developed by Dominik Schrempf.
-    Compiled on February 19, 2021, at 13:42 pm, UTC.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
     
     Usage: slynx [-v|--verbosity VALUE] [-o|--output-file-basename NAME] 
                  [-f|--force] [--no-elynx-file] COMMAND
@@ -84,8 +84,8 @@
     Available options:
       -h,--help                Show this help text
       -V,--version             Show version
-      -v,--verbosity VALUE     Be verbose; one of: Quiet Warning Info
-                               Debug (default: Info)
+      -v,--verbosity VALUE     Be verbose; one of: Quiet Warning Info Debug
+                               (default: Info)
       -o,--output-file-basename NAME
                                Specify base name of output file
       -f,--force               Ignore previous analysis and overwrite existing
@@ -118,11 +118,11 @@
 
 Concatenate multi sequence alignments.
 
-    slynx concatenate --help
+    stack exec slynx -- concatenate --help
 
-    ELynx Suite version 0.5.0.2.
+    ELynx Suite version 0.5.1.0.
     Developed by Dominik Schrempf.
-    Compiled on February 19, 2021, at 13:42 pm, UTC.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
     
     Usage: slynx concatenate (-a|--alphabet NAME) INPUT-FILE
       Concatenate sequences found in input files.
@@ -139,11 +139,11 @@
 
 Examine sequence with `slynx examine`.
 
-    slynx examine --help
+    stack exec slynx -- examine --help
 
-    ELynx Suite version 0.5.0.2.
+    ELynx Suite version 0.5.1.0.
     Developed by Dominik Schrempf.
-    Compiled on February 19, 2021, at 13:42 pm, UTC.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
     
     Usage: slynx examine (-a|--alphabet NAME) INPUT-FILE [--per-site]
       Examine sequences. If data is a multi sequence alignment, additionally analyze columns.
@@ -161,11 +161,11 @@
 
 Filter sequences with `filer-rows`.
 
-    slynx filter-rows --help
+    stack exec slynx -- filter-rows --help
 
-    ELynx Suite version 0.5.0.2.
+    ELynx Suite version 0.5.1.0.
     Developed by Dominik Schrempf.
-    Compiled on February 19, 2021, at 13:42 pm, UTC.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
     
     Usage: slynx filter-rows (-a|--alphabet NAME) INPUT-FILE [--longer-than LENGTH] 
                              [--shorter-than LENGTH] [--standard-characters]
@@ -184,11 +184,11 @@
 
 Filter columns of multi sequence alignments with `filter-columns`.
 
-    slynx filter-columns --help
+    stack exec slynx -- filter-columns --help
 
-    ELynx Suite version 0.5.0.2.
+    ELynx Suite version 0.5.1.0.
     Developed by Dominik Schrempf.
-    Compiled on February 19, 2021, at 13:42 pm, UTC.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
     
     Usage: slynx filter-columns (-a|--alphabet NAME) INPUT-FILE 
                                 [--standard-chars DOUBLE]
@@ -208,11 +208,11 @@
 
 Simulate sequences with `slynx simulate`.
 
-    slynx simulate --help
+    stack exec slynx -- simulate --help
 
-    ELynx Suite version 0.5.0.2.
+    ELynx Suite version 0.5.1.0.
     Developed by Dominik Schrempf.
-    Compiled on February 19, 2021, at 13:42 pm, UTC.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
     
     Usage: slynx simulate (-t|--tree-file Name) [-s|--substitution-model MODEL] 
                           [-m|--mixture-model MODEL] [-e|--edm-file NAME] 
@@ -287,11 +287,11 @@
 
 Sub-sample columns from multi sequence alignments.
 
-    slynx sub-sample --help
+    stack exec slynx -- sub-sample --help
 
-    ELynx Suite version 0.5.0.2.
+    ELynx Suite version 0.5.1.0.
     Developed by Dominik Schrempf.
-    Compiled on February 19, 2021, at 13:42 pm, UTC.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
     
     Usage: slynx sub-sample (-a|--alphabet NAME) INPUT-FILE
                             (-n|--number-of-sites INT)
@@ -317,11 +317,11 @@
 
 Translate sequences.
 
-    slynx translate --help
+    stack exec slynx -- translate --help
 
-    ELynx Suite version 0.5.0.2.
+    ELynx Suite version 0.5.1.0.
     Developed by Dominik Schrempf.
-    Compiled on February 19, 2021, at 13:42 pm, UTC.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
     
     Usage: slynx translate (-a|--alphabet NAME) INPUT-FILE (-r|--reading-frame INT)
                            (-u|--universal-code CODE)
@@ -342,11 +342,11 @@
 
 Handle phylogenetic trees in Newick format.
 
-    tlynx --help | head -n -16
+    stack exec tlynx -- --help | head -n -16
 
-    ELynx Suite version 0.5.0.2.
+    ELynx Suite version 0.5.1.0.
     Developed by Dominik Schrempf.
-    Compiled on February 19, 2021, at 13:42 pm, UTC.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
     
     Usage: tlynx [-v|--verbosity VALUE] [-o|--output-file-basename NAME] 
                  [-f|--force] [--no-elynx-file] COMMAND
@@ -355,8 +355,8 @@
     Available options:
       -h,--help                Show this help text
       -V,--version             Show version
-      -v,--verbosity VALUE     Be verbose; one of: Quiet Warning Info
-                               Debug (default: Info)
+      -v,--verbosity VALUE     Be verbose; one of: Quiet Warning Info Debug
+                               (default: Info)
       -o,--output-file-basename NAME
                                Specify base name of output file
       -f,--force               Ignore previous analysis and overwrite existing
@@ -382,11 +382,11 @@
 
 Compute distances between phylogenetic trees.
 
-    tlynx compare --help
+    stack exec tlynx -- compare --help
 
-    ELynx Suite version 0.5.0.2.
+    ELynx Suite version 0.5.1.0.
     Developed by Dominik Schrempf.
-    Compiled on February 19, 2021, at 13:42 pm, UTC.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
     
     Usage: tlynx compare [-n|--normalize] [-b|--bipartitions] [-t|--intersect] 
                          [-f|--newick-format FORMAT] NAMES
@@ -407,15 +407,90 @@
       -h,--help                Show this help text
 
 
+## Connect
+
+Connect two phylogenetic tree in all ways (possibly honoring constraints).
+
+    stack exec tlynx -- connect --help
+
+    ELynx Suite version 0.5.1.0.
+    Developed by Dominik Schrempf.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
+    
+    Usage: tlynx connect [-f|--newick-format FORMAT] [-c|--contraints CONSTRAINTS]
+                         TREE-FILE-A TREE-FILE-B
+      Connect two phylogenetic trees in all ways (possibly honoring constraints).
+    
+    Available options:
+      -h,--help                Show this help text
+      -V,--version             Show version
+      -f,--newick-format FORMAT
+                               Newick tree format: Standard, IqTree, or RevBayes;
+                               default: Standard; for detailed help, see 'tlynx
+                               --help'
+      -c,--contraints CONSTRAINTS
+                               File containing one or more Newick trees to be used
+                               as constraints
+      TREE-FILE-A              File containing the first Newick tree
+      TREE-FILE-B              File containing the second Newick tree
+      -h,--help                Show this help text
+
+
+## Distancce
+
+Compute distances between many phylogenetic trees.
+
+    stack exec tlynx -- distance --help
+
+    ELynx Suite version 0.5.1.0.
+    Developed by Dominik Schrempf.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
+    
+    Usage: tlynx distance (-d|--distance MEASURE) [-n|--normalize] [-t|--intersect] 
+                          [-s|--summary-statistics] 
+                          [-m|--master-tree-file MASTER-TREE-File] 
+                          [-f|--newick-format FORMAT] [INPUT-FILES]
+      Compute distances between many phylogenetic trees.
+    
+    Available options:
+      -h,--help                Show this help text
+      -V,--version             Show version
+      -d,--distance MEASURE    Type of distance to calculate (available distance
+                               measures are listed below)
+      -n,--normalize           Normalize trees before distance calculation; only
+                               affect distances depending on branch lengths
+      -t,--intersect           Compare intersections; i.e., before comparison, drop
+                               leaves that are not present in the other tree
+      -s,--summary-statistics  Report summary statistics only
+      -m,--master-tree-file MASTER-TREE-File
+                               Compare all trees to the tree in the master tree
+                               file.
+      -f,--newick-format FORMAT
+                               Newick tree format: Standard, IqTree, or RevBayes;
+                               default: Standard; for detailed help, see 'tlynx
+                               --help'
+      INPUT-FILES              Read tree(s) from INPUT-FILES; if more files are
+                               given, one tree is expected per file
+      -h,--help                Show this help text
+    
+    Distance measures:
+      symmetric                Symmetric distance (Robinson-Foulds distance).
+      incompatible-split[VAL]  Incompatible split distance. Collapse branches with (normalized)
+                               support less than 0.0<=VAL<=1.0 before distance calculation;
+                               if, let's say, VAL>0.7, only well supported differences contribute
+                               to the total distance.
+      branch-score             Branch score distance.
+
+
 ## Examine
 
 Compute summary statistics of phylogenetic trees.
 
-    tlynx examine --help
+    stack exec tlynx -- examine --help
 
-    ELynx Suite version 0.5.0.2.
+    ELynx Suite version 0.5.1.0.
     Developed by Dominik Schrempf.
-    Compiled on February 19, 2021, at 13:42 pm, UTC.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
     
     Usage: tlynx examine INPUT-FILE [-f|--newick-format FORMAT]
       Compute summary statistics of phylogenetic trees.
@@ -431,15 +506,44 @@
       -h,--help                Show this help text
 
 
+## Shuffle
+
+Shuffle a phylogenetic tree (keep coalescent times, but shuffle topology and
+leaves).
+
+    stack exec tlynx -- shuffle --help
+
+    ELynx Suite version 0.5.1.0.
+    Developed by Dominik Schrempf.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
+    
+    Usage: tlynx shuffle [-f|--newick-format FORMAT] [-n|--replicates N] TREE-FILE 
+                         [-S|--seed [INT]]
+      Shuffle a phylogenetic tree (keep coalescent times, but shuffle topology and leaves).
+    
+    Available options:
+      -h,--help                Show this help text
+      -V,--version             Show version
+      -f,--newick-format FORMAT
+                               Newick tree format: Standard, IqTree, or RevBayes;
+                               default: Standard; for detailed help, see 'tlynx
+                               --help'
+      -n,--replicates N        Number of trees to generate
+      TREE-FILE                File containing a Newick tree
+      -S,--seed [INT]          Seed for random number generator; list of 32 bit
+                               integers with up to 256 elements (default: random)
+      -h,--help                Show this help text
+
+
 ## Simulate
 
 Simulate phylogenetic trees using birth and death processes.
 
-    tlynx simulate --help
+    stack exec tlynx -- simulate --help
 
-    ELynx Suite version 0.5.0.2.
+    ELynx Suite version 0.5.1.0.
     Developed by Dominik Schrempf.
-    Compiled on February 19, 2021, at 13:42 pm, UTC.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
     
     Usage: tlynx simulate (-t|--nTrees INT) (-n|--nLeaves INT) PROCESS 
                           [-u|--sub-sample DOUBLE] [-s|--summary-statistics] 
@@ -472,11 +576,11 @@
 
 Validate and (optionally) redo past ELynx analyses.
 
-    elynx --help | head -n -16
+    stack exec elynx -- --help | head -n -16
 
-    ELynx Suite version 0.5.0.2.
+    ELynx Suite version 0.5.1.0.
     Developed by Dominik Schrempf.
-    Compiled on February 19, 2021, at 13:42 pm, UTC.
+    Compiled on June 12, 2021, at 14:54 pm, UTC.
     
     Usage: elynx COMMAND
       Validate and redo past ELynx analyses
diff --git a/bench/Length.hs b/bench/Length.hs
--- a/bench/Length.hs
+++ b/bench/Length.hs
@@ -22,7 +22,14 @@
 import ELynx.Tree.Length
 
 lengthSumFoldl' :: [Length] -> Length
-lengthSumFoldl' = foldl' (\x y -> toLength "lengthSumFoldl'" $ fromLength x + fromLength y) 0
+lengthSumFoldl' =
+  foldl'
+    ( \x y ->
+        either (error . (<>) "lengthSumFoldl'") id $
+          toLength $
+            fromLength x + fromLength y
+    )
+    0
 
 lengthSumFoldl'Unsafe :: [Length] -> Length
 lengthSumFoldl'Unsafe = foldl' (\x y -> toLengthUnsafe $ fromLength x + fromLength y) 0
diff --git a/elynx-tree.cabal b/elynx-tree.cabal
--- a/elynx-tree.cabal
+++ b/elynx-tree.cabal
@@ -1,6 +1,6 @@
 cabal-version:      2.2
 name:               elynx-tree
-version:            0.5.0.2
+version:            0.5.1.0
 license:            GPL-3.0-or-later
 license-file:       LICENSE
 copyright:          Dominik Schrempf (2020)
@@ -76,7 +76,7 @@
     default-language: Haskell2010
     ghc-options:      -Wall -Wunused-packages
     build-depends:
-        aeson >=1.5.5.1,
+        aeson >=1.5.6.0,
         attoparsec >=0.13.2.5,
         base >=4.14.1.0,
         bytestring >=0.10.12.0,
@@ -84,8 +84,8 @@
         containers >=0.6.2.1,
         deepseq >=1.4.4.0,
         double-conversion >=2.0.2.0,
-        elynx-nexus >=0.5.0.2,
-        math-functions >=0.3.4.1,
+        elynx-nexus >=0.5.1.0,
+        math-functions >=0.3.4.2,
         mwc-random >=0.14.0.0,
         parallel >=3.2.2.0,
         primitive >=0.7.1.0,
@@ -118,9 +118,9 @@
         base >=4.14.1.0,
         bytestring >=0.10.12.0,
         containers >=0.6.2.1,
-        elynx-tools >=0.5.0.2,
+        elynx-tools >=0.5.1.0,
         elynx-tree -any,
-        hspec >=2.7.8
+        hspec >=2.7.10
 
 benchmark tree-bench
     type:             exitcode-stdio-1.0
@@ -138,8 +138,8 @@
     build-depends:
         base >=4.7 && <5,
         criterion >=1.5.9.0,
-        elynx-tools >=0.5.0.2,
+        elynx-tools >=0.5.1.0,
         elynx-tree -any,
-        microlens >=0.4.11.2,
+        microlens >=0.4.12.0,
         mwc-random >=0.14.0.0,
         parallel >=3.2.2.0
diff --git a/src/ELynx/Topology/Rooted.hs b/src/ELynx/Topology/Rooted.hs
--- a/src/ELynx/Topology/Rooted.hs
+++ b/src/ELynx/Topology/Rooted.hs
@@ -28,6 +28,7 @@
     Forest,
     fromTree,
     fromLabeledTree,
+    toLabeledTreeWith,
 
     -- * Access leaves, branches and labels
     leaves,
@@ -128,11 +129,17 @@
 fromTree (T.Node lb []) = Leaf lb
 fromTree (T.Node _ xs) = Node $ fromTree <$> N.fromList xs
 
--- | Convert a rooted, labeled rose tree to a rooted topology. Branch labels and
--- internal node labels are lost.
+-- | Convert a rooted, branch-labeled rose tree to a rooted topology. Branch
+-- labels and internal node labels are lost.
 fromLabeledTree :: R.Tree e a -> Topology a
 fromLabeledTree (R.Node _ lb []) = Leaf lb
 fromLabeledTree (R.Node _ _ xs) = Node $ fromLabeledTree <$> N.fromList xs
+
+-- | Convert a rooted topology to a rooted, branch-labeled rose tree. Use the
+-- given node label at internal nodes.
+toLabeledTreeWith :: a -> Topology a -> R.Tree () a
+toLabeledTreeWith _ (Leaf lb) = R.Node () lb []
+toLabeledTreeWith x (Node ts) = R.Node () x $ map (toLabeledTreeWith x) $ N.toList ts
 
 -- TODO: Maybe use foldr similar to 'flatten'.
 -- | Set of leaves.
diff --git a/src/ELynx/Tree.hs b/src/ELynx/Tree.hs
--- a/src/ELynx/Tree.hs
+++ b/src/ELynx/Tree.hs
@@ -74,14 +74,3 @@
 
 --   external        :: n -> Bool
 --   external   = not . internal
-
--- -- -- | Glue branches together, so that one new tree emerges. It's root node is
--- -- -- new, the sub-forest has to be given (a list of trees).
--- -- glue :: (NodeType c)
--- --      => PhyloLabel a b c       -- ^ New root node.
--- --      -> [PhyloTree a b c]      -- ^ Sub-forest.
--- --      -> PhyloTree a b c
--- -- glue s@(PhyloLabel _ _ n) ts
--- --   | extant n  = error "Root node cannot be of type 'Exant'."
--- --   | extinct n = error "Root node cannot be of type 'Extinct'."
--- --   | otherwise = Node s ts
diff --git a/src/ELynx/Tree/Distribution/BirthDeath.hs b/src/ELynx/Tree/Distribution/BirthDeath.hs
--- a/src/ELynx/Tree/Distribution/BirthDeath.hs
+++ b/src/ELynx/Tree/Distribution/BirthDeath.hs
@@ -83,7 +83,7 @@
     res
   | otherwise =
     error $
-      "PointProcess.quantile: p must be in range [0,1] but got "
+      "BirthDeath.quantile: p must be in range [0,1] but got "
         ++ show p
         ++ "."
   where
diff --git a/src/ELynx/Tree/Import/Newick.hs b/src/ELynx/Tree/Import/Newick.hs
--- a/src/ELynx/Tree/Import/Newick.hs
+++ b/src/ELynx/Tree/Import/Newick.hs
@@ -20,24 +20,15 @@
 -- In particular, no conversion from _ to (space) is done right now.
 --
 -- For a description of rooted 'Tree's, please see the 'ELynx.Tree.Rooted'
---
--- Code snippet:
---
--- @
--- import Data.Attoparsec.ByteString
--- import ELynx.Tree
---
--- getOneNewick = either error id . parseOnly (oneNewick Standard)
--- @
 module ELynx.Tree.Import.Newick
   ( NewickFormat (..),
-    describeNewickFormat,
     newick,
-    parseNewick,
     oneNewick,
     parseOneNewick,
+    readOneNewick,
     someNewick,
     parseSomeNewick,
+    readSomeNewick,
   )
 where
 
@@ -56,10 +47,14 @@
 
 -- | Newick tree format.
 --
--- >>> unlines $ map (("- " <>) . description) (allValues :: [NewickFormat])
--- - Standard: Branch support values are stored in square brackets after branch lengths.
--- - IqTree:   Branch support values are stored as node names after the closing bracket of forests.
--- - RevBayes: Key-value pairs is provided in square brackets after node names as well as branch lengths. XXX: Key value pairs are ignored at the moment.
+-- - Standard: Branch support values are stored in square brackets after branch
+--   lengths.
+--
+-- - IqTree: Branch support values are stored as node names after the closing
+--   bracket of forests.
+--
+-- - RevBayes: Key-value pairs are provided in square brackets after node names
+--   as well as branch lengths. XXX: Key value pairs are ignored at the moment.
 data NewickFormat = Standard | IqTree | RevBayes
   deriving (Eq, Show, Read, Bounded, Enum, Generic)
 
@@ -67,56 +62,38 @@
 
 instance ToJSON NewickFormat
 
--- | Short description of the supported Newick formats.
-describeNewickFormat :: NewickFormat -> String
-describeNewickFormat Standard =
-  "Standard: Branch support values are stored in square brackets after branch lengths."
-describeNewickFormat IqTree =
-  "IqTree:   Branch support values are stored as node names after the closing bracket of forests."
-describeNewickFormat RevBayes =
-  "RevBayes: Key-value pairs is provided in square brackets after node names as well as branch lengths. XXX: Key value pairs are ignored at the moment."
-
 -- | Newick tree parser. Also succeeds when more trees follow.
 newick :: NewickFormat -> Parser (Tree Phylo Name)
-newick Standard = newickStandard
-newick IqTree = newickIqTree
-newick RevBayes = newickRevBayes
-
--- | See 'newick'.
-parseNewick :: NewickFormat -> BS.ByteString -> Tree Phylo Name
-parseNewick f = either error id . parseOnly (newick f)
+newick f = case f of
+  Standard -> p tree
+  IqTree -> p treeIqTree
+  RevBayes -> newickRevBayes
+  where
+    p t = skipWhile isSpace *> t <* char ';' <* skipWhile isSpace <?> "newick"
 
 -- | One Newick tree parser. Fails when end of input is not reached.
 oneNewick :: NewickFormat -> Parser (Tree Phylo Name)
-oneNewick Standard = oneNewickStandard
-oneNewick IqTree = oneNewickIqTree
-oneNewick RevBayes = oneNewickRevBayes
+oneNewick f = newick f <* endOfInput <?> "oneNewick"
 
 -- | See 'oneNewick'.
-parseOneNewick :: NewickFormat -> BS.ByteString -> Tree Phylo Name
-parseOneNewick f = either error id . parseOnly (oneNewick f)
+parseOneNewick :: NewickFormat -> BS.ByteString -> Either String (Tree Phylo Name)
+parseOneNewick f = parseOnly (oneNewick f)
 
+-- | See 'oneNewick'; may fail with 'error'.
+readOneNewick :: NewickFormat -> FilePath -> IO (Tree Phylo Name)
+readOneNewick f fn = BS.readFile fn >>= (either error pure . parseOneNewick f)
+
 -- | One or more Newick trees parser.
 someNewick :: NewickFormat -> Parser (Forest Phylo Name)
-someNewick Standard = someNewickStandard
-someNewick IqTree = someNewickIqTree
-someNewick RevBayes = someNewickRevBayes
+someNewick f = some (newick f) <* endOfInput <?> "someNewick"
 
 -- | See 'someNewick'.
-parseSomeNewick :: NewickFormat -> BS.ByteString -> [Tree Phylo Name]
-parseSomeNewick f = either error id . parseOnly (someNewick f)
-
--- Parse a single Newick tree. Also succeeds when more trees follow.
-newickStandard :: Parser (Tree Phylo Name)
-newickStandard = skipWhile isSpace *> tree <* char ';' <* skipWhile isSpace <?> "newickStandard"
-
--- Parse a single Newick tree. Fails when end of file is not reached.
-oneNewickStandard :: Parser (Tree Phylo Name)
-oneNewickStandard = newickStandard <* endOfInput <?> "oneNewickStandard"
+parseSomeNewick :: NewickFormat -> BS.ByteString -> Either String [Tree Phylo Name]
+parseSomeNewick f = parseOnly (someNewick f)
 
--- Parse one ore more Newick trees until end of file.
-someNewickStandard :: Parser (Forest Phylo Name)
-someNewickStandard = some newickStandard <* endOfInput <?> "someNewickStandard"
+-- | See 'someNewick'; may fail with 'error'.
+readSomeNewick :: NewickFormat -> FilePath -> IO [Tree Phylo Name]
+readSomeNewick f fn = BS.readFile fn >>= (either error pure . parseSomeNewick f)
 
 tree :: Parser (Tree Phylo Name)
 tree = branched <|> leaf <?> "tree"
@@ -148,40 +125,43 @@
 name = Name . BL.fromStrict <$> takeWhile nameChar <?> "name"
 
 phylo :: Parser Phylo
-phylo = Phylo <$> optional branchLength <*> optional branchSupport <?> "phylo"
+phylo = Phylo <$> optional branchLengthStandard <*> optional branchSupportStandard <?> "phylo"
 
 -- Branch length.
-branchLength :: Parser Length
-branchLength = do
+branchLengthSimple :: Parser Length
+branchLengthSimple = (<?> "branchLengthSimple") $ do
+  l <- double <?> "branchLengthSimple; double"
+  case toLength l of
+    Left e -> fail e
+    Right pl -> pure pl
+
+-- Branch length.
+branchLengthStandard :: Parser Length
+branchLengthStandard = do
   _ <- char ':' <?> "branchLengthDelimiter"
-  l <- double <?> "branchLength"
-  return $ toLength "branchLength" l
+  branchLengthSimple
 
-branchSupport :: Parser Support
-branchSupport =
+branchSupportSimple :: Parser Support
+branchSupportSimple = (<?> "branchSupportSimple") $
   do
-    _ <- char '[' <?> "branchSupportBegin"
-    s <- double <?> "branchSupport"
-    _ <- char ']' <?> "branchSupportEnd"
-    return $ toSupport "branchSupport" s
+    s <- double <?> "branchSupportSimple; double"
+    case toSupport s of
+      Left e -> fail e
+      Right ps -> pure ps
 
+branchSupportStandard :: Parser Support
+branchSupportStandard = (<?> "branchSupportStandard") $ do
+  _ <- char '[' <?> "branchSupportBegin"
+  s <- branchSupportSimple
+  _ <- char ']' <?> "branchSupportEnd"
+  return s
+
 --------------------------------------------------------------------------------
 -- IQ-TREE.
-
+--
 -- IQ-TREE stores the branch support as node names after the closing bracket of
 -- a forest. Parse a single Newick tree. Also succeeds when more trees follow.
-newickIqTree :: Parser (Tree Phylo Name)
-newickIqTree = skipWhile isSpace *> treeIqTree <* char ';' <* skipWhile isSpace <?> "newickIqTree"
 
--- See 'newickIqTree'. Parse a single Newick tree. Fails when end of file is not
--- reached.
-oneNewickIqTree :: Parser (Tree Phylo Name)
-oneNewickIqTree = newickIqTree <* endOfInput <?> "oneNewickIqTree"
-
--- See 'newickIqTree'. Parse one ore more Newick trees until end of file.
-someNewickIqTree :: Parser (Forest Phylo Name)
-someNewickIqTree = some newickIqTree <* endOfInput <?> "someNewickIqTree"
-
 -- IQ-TREE stores the branch support as node names after the closing bracket of a forest.
 treeIqTree :: Parser (Tree Phylo Name)
 treeIqTree = branchedIqTree <|> leaf <?> "treeIqTree"
@@ -190,11 +170,10 @@
 branchedIqTree :: Parser (Tree Phylo Name)
 branchedIqTree = (<?> "branchedIqTree") $ do
   f <- forestIqTree
-  ms <- optional double
-  let s = toSupport "branchedIqTree" <$> ms
+  ms <- optional branchSupportSimple
   n <- name
-  b <- optional branchLength
-  return $ Node (Phylo b s) n f
+  mb <- optional branchLengthStandard
+  return $ Node (Phylo mb ms) n f
 
 -- IQ-TREE stores the branch support as node names after the closing bracket of a forest.
 forestIqTree :: Parser (Forest Phylo Name)
@@ -220,15 +199,6 @@
     <* char ';'
     <* skipWhile isSpace <?> "newickRevBayes"
 
--- See 'newickRevBayes'. Parse a single Newick tree. Fails when end of file is
--- not reached.
-oneNewickRevBayes :: Parser (Tree Phylo Name)
-oneNewickRevBayes = newickRevBayes <* endOfInput <?> "oneNewickRevBayes"
-
--- See 'newickRevBayes'. Parse one ore more Newick trees until end of file.
-someNewickRevBayes :: Parser (Forest Phylo Name)
-someNewickRevBayes = some newickRevBayes <* endOfInput <?> "someNewickRevBayes"
-
 treeRevBayes :: Parser (Tree Phylo Name)
 treeRevBayes = branchedRevBayes <|> leafRevBayes <?> "treeRevBayes"
 
@@ -250,7 +220,7 @@
 nameRevBayes = name <* optional brackets <?> "nameRevBayes"
 
 branchLengthRevBayes :: Parser Length
-branchLengthRevBayes = branchLength <* optional brackets <?> "branchLengthRevBayes"
+branchLengthRevBayes = branchLengthStandard <* optional brackets <?> "branchLengthRevBayes"
 
 leafRevBayes :: Parser (Tree Phylo Name)
 leafRevBayes = (<?> "leafRevBayes") $ do
diff --git a/src/ELynx/Tree/Length.hs b/src/ELynx/Tree/Length.hs
--- a/src/ELynx/Tree/Length.hs
+++ b/src/ELynx/Tree/Length.hs
@@ -21,7 +21,6 @@
     Length (fromLength),
     toLength,
     toLengthUnsafe,
-    checkLength,
     HasLength (..),
     height,
     rootHeight,
@@ -88,19 +87,15 @@
   setLen = const
   modLen f = f
 
--- | If negative, call 'error' indicating the calling function name.
-toLength :: String -> Double -> Length
-toLength s x
-  | x < 0 = error $ s ++ ": Length is negative: " ++ show x ++ "."
-  | otherwise = Length x
+-- | Return 'Left' if negative.
+toLength :: Double -> Either String Length
+toLength x
+  | x < 0 = Left $ "Length is negative: " ++ show x ++ "."
+  | otherwise = Right $ Length x
 
 -- | Do not check if value is negative.
 toLengthUnsafe :: Double -> Length
 toLengthUnsafe = Length
-
--- | If negative, call 'error' with given calling function name.
-checkLength :: String -> Length -> Length
-checkLength s = toLength s . fromLength
 
 -- | A data type with measurable and modifiable values.
 class HasLength e where
diff --git a/src/ELynx/Tree/Parallel.hs b/src/ELynx/Tree/Parallel.hs
--- a/src/ELynx/Tree/Parallel.hs
+++ b/src/ELynx/Tree/Parallel.hs
@@ -9,9 +9,17 @@
 -- Portability :  portable
 --
 -- Creation date: Mon Sep  7 13:36:45 2020.
+--
+-- Parallel evaluation up to a given layer. By convention layer 0 only has one
+-- element: the root node. The layer 1 includes the daughter nodes of the root
+-- node, and so on.
+--
+-- The layer to which a node belongs should not be confused with the 'depth' of
+-- a tree.
 module ELynx.Tree.Parallel
   ( parTree,
     parBranchFoldMap,
+    parBranchFoldMapWithLayer,
     parNodeFoldMap,
   )
 where
@@ -32,31 +40,46 @@
 -- Evaluate the sub trees up to given layer in parallel.
 parTree :: (NFData e, NFData a) => Int -> Strategy (Tree e a)
 parTree n t@(Node br lb ts)
-  | n == 0 = rdeepseq t
   | n == 1 = do
     ts' <- myParList rdeepseq ts
     return $ Node br lb ts'
   | n >= 2 = do
     ts' <- myParList (parTree (n -1)) ts
     return $ Node br lb ts'
-  | otherwise = error "parTree: n is negative."
+  | otherwise = rdeepseq t
 
 branchFoldMap :: (e -> f) -> (f -> f -> f) -> Tree e a -> f
 branchFoldMap f op (Node br _ ts) = foldl' op (f br) $ map (branchFoldMap f op) ts
 
 -- | Map and fold over branches. Evaluate the sub trees up to given layer in parallel.
 parBranchFoldMap :: NFData f => Int -> (e -> f) -> (f -> f -> f) -> Tree e a -> f
-parBranchFoldMap 0 f op t = branchFoldMap f op t
-parBranchFoldMap n f op (Node br _ ts)
+parBranchFoldMap n f op t@(Node br _ ts)
   | n >= 1 = foldl' op (f br) (map (parBranchFoldMap (n - 1) f op) ts `using` myParList rdeepseq)
-  | otherwise = error "parBranchFoldMap: n is negative."
+  | otherwise = branchFoldMap f op t
 
+branchFoldMapWithLayer :: Int -> (Int -> e -> f) -> (f -> f -> f) -> Tree e a -> f
+branchFoldMapWithLayer d f op (Node br _ ts) =
+  foldl' op (f d br) (map (branchFoldMapWithLayer (d + 1) f op) ts)
+
+-- | Map and fold over branches.
+--
+-- The used function has access to the layer of the node to which the handled
+-- branch is attached to.
+--
+-- Evaluate the sub trees up to given layer in parallel.
+parBranchFoldMapWithLayer :: NFData f => Int -> (Int -> e -> f) -> (f -> f -> f) -> Tree e a -> f
+parBranchFoldMapWithLayer = go 0
+  where
+    go d n f op t@(Node br _ ts)
+      | n >= 1 =
+        foldl' op (f d br) (map (go (d + 1) (n - 1) f op) ts `using` myParList rdeepseq)
+      | otherwise = branchFoldMapWithLayer d f op t
+
 nodeFoldMap :: (a -> b) -> (b -> b -> b) -> Tree e a -> b
 nodeFoldMap f op (Node _ lb ts) = foldl' op (f lb) $ map (nodeFoldMap f op) ts
 
 -- | Map and fold over nodes. Evaluate the sub trees up to given layer in parallel.
 parNodeFoldMap :: NFData b => Int -> (a -> b) -> (b -> b -> b) -> Tree e a -> b
-parNodeFoldMap 0 f op t = nodeFoldMap f op t
-parNodeFoldMap n f op (Node _ lb ts)
+parNodeFoldMap n f op t@(Node _ lb ts)
   | n >= 1 = foldl' op (f lb) (map (parNodeFoldMap (n - 1) f op) ts `using` myParList rdeepseq)
-  | otherwise = error "parNodeFoldMap: n is negative."
+  | otherwise = nodeFoldMap f op t
diff --git a/src/ELynx/Tree/Phylogeny.hs b/src/ELynx/Tree/Phylogeny.hs
--- a/src/ELynx/Tree/Phylogeny.hs
+++ b/src/ELynx/Tree/Phylogeny.hs
@@ -32,7 +32,7 @@
 --
 -- 4. Uniqueness of the leaves is not ensured by the data type, but has to be
 -- checked at runtime. Functions relying on the tree to have unique leaves do
--- perform this check, and return 'Left' with an error message, if the tree has
+-- perform this check, and return 'Left' with a message, if the tree has
 -- duplicate leaves.
 --
 -- Note: 'Tree's are rooted.
@@ -94,8 +94,6 @@
 -- | The equality check is slow because the order of children is considered to
 -- be arbitrary.
 --
--- NOTE: The equality check is only meaningful if the trees have unique leaves.
---
 -- Return 'Left' if a tree does not have unique leaves.
 equal :: (Eq e, Eq a, Ord a) => Tree e a -> Tree e a -> Either String Bool
 equal tL tR
@@ -168,9 +166,10 @@
 -- If the current root node is multifurcating, a bifurcating root node with the
 -- empty label is introduced by 'split'ting the leftmost branch. The 'Monoid'
 -- instance of the node label and the 'Splittable' instance of the branch length
--- are used. Note that in this case, the degree of the former root node is
--- decreased by one!
+-- are used.
 --
+-- NOTE: In this case, the degree of the former root node is decreased by one!
+--
 -- Given that the root note is bifurcating, the root node is moved to the
 -- required position specified by the outgroup.
 --
@@ -212,35 +211,37 @@
 midpoint :: (Semigroup e, Splittable e, HasLength e) => Tree e a -> Either String (Tree e a)
 midpoint (Node _ _ []) = Left "midpoint: Root node is a leaf."
 midpoint (Node _ _ [_]) = Left "midpoint: Root node has degree two."
-midpoint t@(Node _ _ [_, _]) = getMidpoint <$> roots t
+midpoint t@(Node _ _ [_, _]) = roots t >>= getMidpoint
 midpoint _ = Left "midpoint: Root node is multifurcating."
 
-findMinIndex :: Ord a => [a] -> Int
+-- Find the index of the smallest element.
+findMinIndex :: Ord a => [a] -> Either String Int
 findMinIndex (x : xs) = go (0, x) 1 xs
   where
-    go (i, _) _ [] = i
+    go (i, _) _ [] = Right i
     go (i, z) j (y : ys) = if z < y then go (i, z) (j + 1) ys else go (j, y) (j + 1) ys
-findMinIndex [] = error "findMinIndex: Empty list."
+findMinIndex [] = Left "findMinIndex: Empty list."
 
-getMidpoint :: HasLength e => [Tree e a] -> Tree e a
+getMidpoint :: HasLength e => [Tree e a] -> Either String (Tree e a)
 getMidpoint ts = case t of
-  (Node br lb [l, r]) ->
+  Right (Node br lb [l, r]) ->
     let hl = height l
         hr = height r
         dh = (hl - hr) / 2
-     in Node
-          br
-          lb
-          [ applyStem (modLen (subtract dh)) l,
-            applyStem (modLen (+ dh)) r
-          ]
+     in Right $
+          Node
+            br
+            lb
+            [ applyStem (modLen (subtract dh)) l,
+              applyStem (modLen (+ dh)) r
+            ]
   -- Explicitly use 'error' here, because roots is supposed to return trees with
   -- bifurcating root nodes.
-  _ -> error "getMidpoint: Root node is not bifurcating."
+  Right _ -> error "getMidpoint: Root node is not bifurcating; please contact maintainer."
+  Left e -> Left e
   where
     dhs = map getDeltaHeight ts
-    i = findMinIndex dhs
-    t = ts !! i
+    t = (ts !!) <$> findMinIndex dhs
 
 -- find index of minimum; take this tree and move root to the midpoint of the branch
 
@@ -249,7 +250,7 @@
 getDeltaHeight (Node _ _ [l, r]) = abs $ height l - height r
 -- Explicitly use 'error' here, because roots is supposed to return trees with
 -- bifurcating root nodes.
-getDeltaHeight _ = error "getDeltaHeight: Root node is not bifurcating."
+getDeltaHeight _ = error "getDeltaHeight: Root node is not bifurcating; please contact maintainer."
 
 -- | For a rooted tree with a bifurcating root node, get all possible rooted
 -- trees.
@@ -485,5 +486,5 @@
   st <- phyloToSupportTree t
   case zipTreesWith PhyloExplicit const lt st of
     -- Explicit use of error, since this case should not happen.
-    Nothing -> error "toExplicitTree: Can not zip two trees with the same topology."
+    Nothing -> error "toExplicitTree: Can not zip two trees with different topologies; please contact maintainer."
     Just zt -> return zt
diff --git a/src/ELynx/Tree/Rooted.hs b/src/ELynx/Tree/Rooted.hs
--- a/src/ELynx/Tree/Rooted.hs
+++ b/src/ELynx/Tree/Rooted.hs
@@ -79,6 +79,7 @@
 
     -- * Structure
     degree,
+    depth,
     prune,
     dropNodesWith,
     dropLeavesWith,
@@ -320,9 +321,23 @@
 identify :: Traversable t => t a -> t Int
 identify = snd . mapAccumL (\i _ -> (i + 1, i)) (0 :: Int)
 
--- | The degree of the root node.
+-- | Degree of the root node.
+--
+-- The degree of a node is the number of branches attached to the node.
 degree :: Tree e a -> Int
 degree = (+ 1) . length . forest
+
+-- | Depth of a tree.
+--
+-- The [depth of a tree](https://en.wikipedia.org/wiki/Tree-depth) is the
+-- largest number of nodes traversed on a path from the root to a leaf.
+--
+-- By convention, the depth is larger equal 1. That is, the depth of a leaf tree
+-- is 1.
+depth :: Tree e a -> Int
+depth = maximum . go 1
+  where go n (Node _ _ []) = [n]
+        go n (Node _ _ xs) = concatMap (go (n+1)) xs
 
 -- | Prune degree two nodes.
 --
diff --git a/src/ELynx/Tree/Splittable.hs b/src/ELynx/Tree/Splittable.hs
--- a/src/ELynx/Tree/Splittable.hs
+++ b/src/ELynx/Tree/Splittable.hs
@@ -14,10 +14,9 @@
   )
 where
 
--- | A data type that can be combined using '<>' and split into one out of two
--- equal entities.
+-- | A data type that can be split into two equal entities.
 --
--- The following equality should hold:
+-- For 'Semigroup's, the following equality should hold:
 --
 -- @
 -- split x <> split x = x
diff --git a/src/ELynx/Tree/Support.hs b/src/ELynx/Tree/Support.hs
--- a/src/ELynx/Tree/Support.hs
+++ b/src/ELynx/Tree/Support.hs
@@ -59,11 +59,10 @@
   setSup = const
   modSup f = f
 
--- | If negative, call 'error' indicating the calling function name.
-toSupport :: String -> Double -> Support
-toSupport s x
-  | x < 0 = error $ s ++ ": Support is negative: " ++ show x ++ "."
-  | otherwise = Support x
+-- | Return 'Left' if negative.
+toSupport :: Double -> Either String Support
+toSupport x | x < 0 = Left $ "Support is negative: " ++ show x ++ "."
+            | otherwise = Right $ Support x
 
 -- | Do not check if value is negative.
 toSupportUnsafe :: Double -> Support
diff --git a/src/ELynx/Tree/Zipper.hs b/src/ELynx/Tree/Zipper.hs
--- a/src/ELynx/Tree/Zipper.hs
+++ b/src/ELynx/Tree/Zipper.hs
@@ -29,9 +29,10 @@
     -- * Paths
     Path,
     goPath,
-    validPath,
     goPathUnsafe,
     getSubTreeUnsafe,
+    isValidPath,
+    isLeafPath,
 
     -- * Modification
     insertTree,
@@ -170,10 +171,16 @@
 goPath pos pth = foldlM (flip goChild) pth pos
 
 -- | Check if a path is valid in that it leads to a node on a tree.
-validPath :: Tree e a -> Path -> Either String Path
-validPath t p = case goPath p (fromTree t) of
-  Nothing -> Left "validPath: Path is invalid."
-  Just _ -> Right p
+isValidPath :: Tree e a -> Path -> Bool
+isValidPath t p = case goPath p (fromTree t) of
+  Nothing -> False
+  Just _ -> True
+
+-- | Check if a path leads to a leaf.
+isLeafPath :: Tree e a -> Path -> Bool
+isLeafPath t p = case goPath p (fromTree t) of
+  Nothing -> False
+  Just pos -> null $ forest (current pos)
 
 -- | Got to node with given path.
 --