diff --git a/.travis.yml b/.travis.yml
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,6 @@
 # This Travis job script has been generated by a script via
 #
-#   runghc make_travis_yml_2.hs '-o' '.travis.yml' '--irc-channel=irc.freenode.org#haskell-lens' '--no-no-tests-no-bench' '--no-installed' 'cabal.project'
+#   runghc make_travis_yml_2.hs '-o' '.travis.yml' '--ghc-head' '--irc-channel=irc.freenode.org#haskell-lens' '--no-no-tests-no-bench' '--no-unconstrained' 'cabal.project'
 #
 # For more information, see https://github.com/hvr/multi-ghc-travis
 #
@@ -44,25 +44,28 @@
 
 matrix:
   include:
-    - compiler: "ghc-7.10.3"
+    - compiler: "ghc-8.6.1"
+      env: GHCHEAD=true
+      addons: {apt: {packages: [*apt_packages,cabal-install-head,ghc-8.6.1], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.4.3"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-7.10.3], sources: [hvr-ghc]}}
+      addons: {apt: {packages: [*apt_packages,cabal-install-2.2,ghc-8.4.3], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.2.2"
+    # env: TEST=--disable-tests BENCH=--disable-benchmarks
+      addons: {apt: {packages: [*apt_packages,cabal-install-2.2,ghc-8.2.2], sources: [hvr-ghc]}}
     - compiler: "ghc-8.0.2"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-8.0.2], sources: [hvr-ghc]}}
-    - compiler: "ghc-8.2.2"
+      addons: {apt: {packages: [*apt_packages,cabal-install-2.2,ghc-8.0.2], sources: [hvr-ghc]}}
+    - compiler: "ghc-7.10.3"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-8.2.2], sources: [hvr-ghc]}}
-    - compiler: "ghc-8.4.1"
-      env: GHCHEAD=true
-      addons: {apt: {packages: [*apt_packages,cabal-install-head,ghc-8.4.1], sources: [hvr-ghc]}}
+      addons: {apt: {packages: [*apt_packages,cabal-install-2.2,ghc-7.10.3], sources: [hvr-ghc]}}
     - compiler: "ghc-head"
       env: GHCHEAD=true
       addons: {apt: {packages: [*apt_packages,cabal-install-head,ghc-head], sources: [hvr-ghc]}}
 
   allow_failures:
-    - compiler: "ghc-8.4.1"
     - compiler: "ghc-head"
+    - compiler: "ghc-8.6.1"
 
 before_install:
   - HC=${CC}
@@ -80,7 +83,8 @@
   - BENCH=${BENCH---enable-benchmarks}
   - TEST=${TEST---enable-tests}
   - HADDOCK=${HADDOCK-true}
-  - INSTALLED=${INSTALLED-true}
+  - UNCONSTRAINED=${UNCONSTRAINED-true}
+  - NOINSTALLEDCONSTRAINTS=${NOINSTALLEDCONSTRAINTS-false}
   - GHCHEAD=${GHCHEAD-false}
   - travis_retry cabal update -v
   - "sed -i.bak 's/^jobs:/-- jobs:/' ${HOME}/.cabal/config"
@@ -88,7 +92,8 @@
   # Overlay Hackage Package Index for GHC HEAD: https://github.com/hvr/head.hackage
   - |
     if $GHCHEAD; then
-      sed -i.bak 's/-- allow-newer:.*/allow-newer: *:base, *:template-haskell, *:ghc, *:Cabal/' ${HOME}/.cabal/config
+      sed -i 's/-- allow-newer: .*/allow-newer: *:base/' ${HOME}/.cabal/config
+      for pkg in $($HCPKG list --simple-output); do pkg=$(echo $pkg | sed 's/-[^-]*$//'); sed -i "s/allow-newer: /allow-newer: *:$pkg, /" ${HOME}/.cabal/config; done
 
       echo 'repository head.hackage'                                                        >> ${HOME}/.cabal/config
       echo '   url: http://head.hackage.haskell.org/'                                       >> ${HOME}/.cabal/config
@@ -98,17 +103,22 @@
       echo '              8f79fd2389ab2967354407ec852cbe73f2e8635793ac446d09461ffb99527f6e' >> ${HOME}/.cabal/config
       echo '   key-threshold: 3'                                                            >> ${HOME}/.cabal.config
 
+      grep -Ev -- '^\s*--' ${HOME}/.cabal/config | grep -Ev '^\s*$'
+
       cabal new-update head.hackage -v
     fi
   - grep -Ev -- '^\s*--' ${HOME}/.cabal/config | grep -Ev '^\s*$'
   - "printf 'packages: \".\"\\n' > cabal.project"
-  - cat cabal.project
+  - touch cabal.project.local
+  - "if ! $NOINSTALLEDCONSTRAINTS; then for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/^/constraints: /' | sed 's/-[^-]*$/ installed/' >> cabal.project.local; done; fi"
+  - cat cabal.project || true
+  - cat cabal.project.local || true
   - if [ -f "./configure.ac" ]; then
       (cd "." && autoreconf -i);
     fi
   - rm -f cabal.project.freeze
   - cabal new-build -w ${HC} ${TEST} ${BENCH} --project-file="cabal.project" --dep -j2 all
-  - rm -rf "."/.ghc.environment.* "."/dist
+  - rm -rf .ghc.environment.* "."/dist
   - DISTDIR=$(mktemp -d /tmp/dist-test.XXXX)
 
 # Here starts the actual work to be performed for the package under test;
@@ -120,8 +130,10 @@
   - cd ${DISTDIR} || false
   - find . -maxdepth 1 -name '*.tar.gz' -exec tar -xvf '{}' \;
   - "printf 'packages: ersatz-*/*.cabal\\n' > cabal.project"
-  - cat cabal.project
-
+  - touch cabal.project.local
+  - "if ! $NOINSTALLEDCONSTRAINTS; then for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/^/constraints: /' | sed 's/-[^-]*$/ installed/' >> cabal.project.local; done; fi"
+  - cat cabal.project || true
+  - cat cabal.project.local || true
 
   # build & run tests, build benchmarks
   - cabal new-build -w ${HC} ${TEST} ${BENCH} all
@@ -134,5 +146,5 @@
   - rm -rf ./dist-newstyle
   - if $HADDOCK; then cabal new-haddock -w ${HC} ${TEST} ${BENCH} all; else echo "Skipping haddock generation";fi
 
-# REGENDATA ["-o",".travis.yml","--irc-channel=irc.freenode.org#haskell-lens","--no-no-tests-no-bench","--no-installed","cabal.project"]
+# REGENDATA ["-o",".travis.yml","--ghc-head","--irc-channel=irc.freenode.org#haskell-lens","--no-no-tests-no-bench","--no-unconstrained","cabal.project"]
 # EOF
diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,8 @@
+0.4.3 [2018.07.03]
+------------------
+* Make the test suite compile on GHC 8.6.
+* Allow building with `containers-0.6`.
+
 0.4.2
 -----
 * Add `Semigroup` instances for `Clause` and `Formula`.
diff --git a/ersatz.cabal b/ersatz.cabal
--- a/ersatz.cabal
+++ b/ersatz.cabal
@@ -1,5 +1,5 @@
 name:           ersatz
-version:        0.4.2
+version:        0.4.3
 license:        BSD3
 license-file:   LICENSE
 author:         Edward A. Kmett, Eric Mertens, Johan Kiviniemi
@@ -75,7 +75,11 @@
 
 build-type:     Custom
 cabal-version:  >= 1.10
-tested-with:    GHC == 7.10.3, GHC == 8.0.2, GHC == 8.2.2, GHC == 8.4.1
+tested-with:    GHC == 7.10.3
+              , GHC == 8.0.2
+              , GHC == 8.2.2
+              , GHC == 8.4.3
+              , GHC == 8.6.1
 extra-source-files:
   .ghci
   .gitignore
@@ -178,13 +182,13 @@
     array                >= 0.2      && < 0.6,
     base                 >= 4.8      && < 5,
     bytestring           >= 0.10.4.0 && < 0.12,
-    containers           >= 0.2.0.1  && < 0.6,
+    containers           >= 0.2.0.1  && < 0.7,
     data-default         >= 0.5      && < 0.8,
     lens                 >= 3.8      && < 5,
     mtl                  >= 1.1      && < 2.3,
     process              >= 1.1      && < 1.7,
     semigroups           >= 0.16     && < 1,
-    temporary            >= 1.1      && < 1.3,
+    temporary            >= 1.1      && < 1.4,
     transformers         >= 0.3      && < 0.6,
     unordered-containers == 0.2.*,
     attoparsec
diff --git a/tests/Z001.hs b/tests/Z001.hs
--- a/tests/Z001.hs
+++ b/tests/Z001.hs
@@ -1,7 +1,7 @@
 {-# language KindSignatures, DataKinds, FlexibleContexts #-}
 {-# language GeneralizedNewtypeDeriving #-}
 {-# language TypeFamilies, ScopedTypeVariables #-}
-
+{-# language UndecidableInstances #-}
 {-# language NoMonomorphismRestriction #-}
 
 import Prelude hiding ( not, and, or, (&&), (||) )
@@ -36,7 +36,7 @@
         row f = ( encode f : ) <$> replicateM (d-1) unknown
     m <- (:) <$> row 1 <*> replicateM (d-2) (row 0)
     return $ Restricted $ Matrix
-       $ m ++ encode [ replicate (d-1) 0 ++ [1] ] 
+       $ m ++ encode [ replicate (d-1) 0 ++ [1] ]
 
 class Unknown a where
   unknown :: (MonadState s m, HasSAT s) => m a
@@ -48,12 +48,12 @@
 instance Codec a => Codec (Matrix dim a) where
   type Decoded (Matrix dim a) = Matrix dim (Decoded a)
   decode s (Matrix xss) = Matrix <$> decode s xss
-    
+
 instance (KnownNat dim, Unknown a) => Unknown (Matrix dim a) where
   unknown = do
     let d = fromIntegral $ natVal (Proxy :: Proxy dim)
     Matrix <$> replicateM d (replicateM d unknown)
-    
+
 instance Num a => Num (Matrix dim a) where
   Matrix xss + Matrix yss
     = Matrix $ zipWith (zipWith (+)) xss yss
@@ -81,7 +81,7 @@
 -- Bitvectors of fixed length, with non-overflowing arithmetics
 -- (if overflow occurs, constraint is unsatisfiable)
 
-newtype NBV ( n :: Nat ) = NBV Bits 
+newtype NBV ( n :: Nat ) = NBV Bits
   deriving ( Show, Equatable, Orderable, HasBits )
 
 instance KnownNat w => Unknown (NBV w) where
@@ -91,10 +91,10 @@
 
 positive (NBV (Bits bs)) = or bs
 
-nbv n (Bits bs) = 
+nbv n (Bits bs) =
   let (p : re, post) = splitAt n bs
   in  NBV $ Bits $ Run ( assert (not $ or post) *> return p )
-                 : re 
+                 : re
 
 instance KnownNat n => Num (NBV n) where
   fromInteger = encode