Definition is_midpoint := fun M A B => Bet A M B /\ Cong A M M B.

(* Specifications: *)
Lemma l7_2 : forall M A B, is_midpoint M A B -> is_midpoint M B A.
Lemma l7_3 : forall M A, is_midpoint M A A -> M=A.
Lemma l7_3_2 : forall A, is_midpoint A A A.