fact: Definition cast1 T l : forall (l' : list T) n v, nth_error l n = Some v -> Some v = nth_error (l ++ l') n.
imports: Coq.Lists.List Coq.Arith.PeanoNat Relations RelationClasses ExtLib.Core.RelDec ExtLib.Data.SigT ExtLib.Data.Member ExtLib.Data.ListNth ExtLib.Data.Option ExtLib.Tactics Coq.Classes.Morphisms