Add Parametric Morphism: (@cons alphabet) with signature
  (@eq alphabet) ==> (group_str_equiv) ==> (group_str_equiv) as
  cons_group_str_mor.

Lemma concat_respects_group_str_equiv: forall (S1 S2 T1 T2:group_str), S1 ~~ S2 -> T1 ~~ T2 -> (S1 ++ T1) ~~ (S2 ++ T2).