Add Expressions.removePair function to remove pairs of arguments.

src/Expression.hs

@@ -3,7 +3,8 @@ module Expression (
     -- * Expressions
     Expr, DiffExpr, fromDiffExpr,
     -- ** Operators
-    apply, append, arg, remove, appendSub, appendSubD, filterSub, removeSub,
+    apply, append, arg, remove, removePair,
+    appendSub, appendSubD, filterSub, removeSub,
     -- ** Evaluation
     interpret, interpretPartial, interpretWithStage, interpretDiff,
     -- ** Predicates
@@ -76,6 +77,16 @@ append x = apply (<> x)
 remove :: Eq a => [a] -> DiffExpr [a]
 remove xs = apply $ filter (`notElem` xs)
 
+-- | Remove given pair of elements from a list expression
+-- Example: removePair "-flag" "b" ["-flag", "a", "-flag", "b"] = ["-flag", "a"]
+removePair :: Eq a => a -> a -> DiffExpr [a]
+removePair x y = apply filterPair
+  where
+    filterPair (z1 : z2 : zs) = if x == z1 && y == z2
+                                then filterPair zs
+                                else z1 : filterPair (z2 : zs)
+    filterPair zs = zs
+
 -- | Apply a predicate to an expression
 applyPredicate :: Monoid a => Predicate -> Expr a -> Expr a
 applyPredicate predicate expr = do