(*** minus_plus_plus_l *)
lemma nminus_plus_dx_bi (m) (n) (o): m - n = (m + o) - (n + o).
-#m #n #o <nminus_plus_assoc <nminus_comm //
+#m #n #o <nminus_plus_assoc <nminus_comm_21 //
qed.
(* Helper constructions with nplus ******************************************)