diff --git a/src/Data/Syntax/Expression.hs b/src/Data/Syntax/Expression.hs
index aa9a2c031..e10a8e82f 100644
--- a/src/Data/Syntax/Expression.hs
+++ b/src/Data/Syntax/Expression.hs
@@ -19,3 +19,11 @@ data Not a = Not a
 
 instance Eq1 Not where liftEq = genericLiftEq
 instance Show1 Not where liftShowsPrec = genericLiftShowsPrec
+
+
+-- | Binary addition.
+data Plus a = Plus a a
+  deriving (Eq, Foldable, Functor, Generic1, Show, Traversable)
+
+instance Eq1 Plus where liftEq = genericLiftEq
+instance Show1 Plus where liftShowsPrec = genericLiftShowsPrec