fact: Fixpoint substitutions (xs : list (term * term)) : list (term * term) := match xs with | nil => nil | cons (x, y) xs => match substitution x y with | Some p => cons p (substitutions xs) | None => substitutions xs end end.
imports: Coq.NArith.NArith Coq.FSets.FMaps Coq.micromega.Lia Category.Lib Category.Lib.FMapExt