Add Parametric Morphism: (@app alphabet) with signature
(group_str_equiv) ==> (group_str_equiv) ==> (group_str_equiv) as
concat_mor.

Lemma string_action_takes_concat_to_composition:
forall (S1 S2 S3:group_str),
group_str_action S1 (group_str_action S2 S3) =
group_str_action (S1 ++ S2) S3.