diff --git a/executable/SequentialMain.hs b/executable/SequentialMain.hs
--- a/executable/SequentialMain.hs
+++ b/executable/SequentialMain.hs
@@ -319,9 +319,6 @@
 instance KBO.Weighted Constant where
   argWeight Minimal = 1
   argWeight Constant{..} = con_weight
-instance Arity Constant where
-  arity Minimal = 0
-  arity Constant{..} = con_arity
 
 instance Pretty Constant where
   pPrint Minimal = text "?"
diff --git a/twee.cabal b/twee.cabal
--- a/twee.cabal
+++ b/twee.cabal
@@ -1,5 +1,5 @@
 name:                twee
-version:             2.4
+version:             2.4.1
 synopsis:            An equational theorem prover
 homepage:            http://github.com/nick8325/twee
 license:             BSD3
@@ -38,24 +38,14 @@
   default: False
   manual: True
 
---flag parallel
---  description: Build a special parallel version of Twee.
---  default: False
---  manual: True
-
 executable twee
---  if flag(parallel)
---    main-is: ParallelMain.hs
---    build-depends: async, unix
---    c-sources: executable/link.c
---  else
   main-is: Main.hs
 
   hs-source-dirs:      executable
   other-modules:       SequentialMain
   default-language:    Haskell2010
   build-depends:       base < 5,
-                       twee-lib == 2.4,
+                       twee-lib == 2.4.1,
                        containers,
                        pretty,
                        split,