diff --git a/Number/Peano/Inf.hs b/Number/Peano/Inf.hs
--- a/Number/Peano/Inf.hs
+++ b/Number/Peano/Inf.hs
@@ -51,13 +51,12 @@
     | Succ Nat
     | Inf           
 
+-- Serial instance needed for testing
 instance Serial Nat where
     series = cons0 Zero \/ cons1 Succ \/ cons0 Inf
 
 
-{- | 
-Observable infinity value.
--}
+-- | Observable infinity value.
 infinity :: Nat
 infinity = Inf
 
@@ -168,10 +167,7 @@
 -}
 infixl 6 `infDiff`
 
-infDiff
-    :: Nat             -- ^ n
-    -> Nat             -- ^ m
-    -> Either Nat Nat  -- ^ n >= m: Left (n-m),  n < m: Right (m-n)
+infDiff :: Nat -> Nat -> Either Nat Nat
 
 Inf    `infDiff` _      = Left Inf
 Succ n `infDiff` Succ m = n `infDiff` m
@@ -194,10 +190,7 @@
 -}
 infixl 6 `zeroDiff`
 
-zeroDiff
-    :: Nat             -- ^ n
-    -> Nat             -- ^ m
-    -> Either Nat Nat  -- ^ n >= m: Left (n-m),  n < m: Right (m-n)
+zeroDiff :: Nat -> Nat -> Either Nat Nat
 
 n      `zeroDiff` Zero   = Left  n 
 Zero   `zeroDiff` m      = Right m
@@ -258,7 +251,7 @@
         f i (Succ m) = i' `seq` f i' m  where i' = i+1
         f _ Inf = error "Number.Peano.Inf: fromEnum infinity."
 
-    enumFrom n = enumFromTo n Inf
+    enumFrom = iterate Succ
 
     {- |
             @[inf.. inf] == [inf, inf, inf, ..@
diff --git a/Number/Peano/Inf/Functions.hs b/Number/Peano/Inf/Functions.hs
--- a/Number/Peano/Inf/Functions.hs
+++ b/Number/Peano/Inf/Functions.hs
@@ -2,11 +2,14 @@
     ( minimum
     , maximum
     , length
+    , nodeRank
     ) where
 
 import Number.Peano.Inf
 
-import qualified Prelude as P
+import qualified Data.Set as Set
+
+import qualified Prelude
 import Prelude hiding (minimum, maximum, length)
 
 {- |
@@ -34,11 +37,42 @@
 
 > length (undefined: undefined) >= 1
 -}
-
 length :: [a] -> Nat
 length [] = 0
 length (_:t) = succ (length t)
 
+{- | 
+Rank computation with lazy Peano numbers.
 
+The dependence function represents a graph with multiedges (edges with multiple start nodes).
+@dependence n@ is the list of the start nodes of all multiedges whose end node is @n@.
+
+@nodeRank n@ computes the length of the shortest path to @n@. Note that if @n@ is an end point of a multiedge with no start point,
+then @nodeRank n == 0@.
+
+* If @dependence n == []@ then @nodeRank n == infinity@, because @n@ is not accessable.
+
+* If @any null (dependence n)@ then @nodeRank n == 0@
+
+* Otherwise if @dependence n == [l1, l2, ..]@ then
+    @nodeRank n == 1 + minimum [maximum (map nodeRank l1), maximum (map nodeRank l2), ..]@
+
+If there is an inevitable cycle in the graph, the rank is @infinity@.
+One can observe these cases, due to the observable infinity value.
+-}
+nodeRank 
+    :: Ord n                
+    => (n -> [[n]])         -- ^ dependence function
+    -> n                    -- ^ node
+    -> Nat                  -- ^ rank of the node
+nodeRank dependence node = rank Set.empty node  where
+
+    rank visited node
+        | Set.member node visited   = infinity
+        | otherwise                 = minimum $ map f $ dependence node
+     where
+        visited' = Set.insert node visited
+
+        f = maximum . map (succ . rank visited')
 
 
diff --git a/peano-inf.cabal b/peano-inf.cabal
--- a/peano-inf.cabal
+++ b/peano-inf.cabal
@@ -1,11 +1,12 @@
 name:           peano-inf
-version:        0.6.1
+version:        0.6.2
 synopsis:       Lazy Peano numbers including observable infinity value.
 description:    
     Lazy Peano numbers including observable infinity value.
     .
-    This data type is ideal for lazy list length computation (the infinite value is not needed in this case).
-    For a comparison with other Peano number implementation, see <http://people.inf.elte.hu/divip/peano/>
+    This data type is ideal for lazy list length computation and for serveral graph algroithms.
+    .
+    For a comparison with other Peano number implementations, see <http://people.inf.elte.hu/divip/peano/>
 category:       Data
 author:         Péter Diviánszky <divip@aszt.inf.elte.hu>
 maintainer:     Péter Diviánszky <divip@aszt.inf.elte.hu>
@@ -20,6 +21,7 @@
     ghc-options:    -Wall -fno-warn-overlapping-patterns -fno-warn-incomplete-patterns
     build-depends:
         base,
+        containers,
         lazysmallcheck
 
     exposed-modules: