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

Lemma equiv_strings_have_same_actions:
forall (S1 S2 T:group_str), reduced T ->
S1 ~~ S2 -> group_str_action S1 T =
group_str_action S2 T.