Definition prog_c : com :=
  SKIP.Definition prog_g : com :=
  WHILE BTrue DO
    SKIP
  END.Definition equiv_classes : list (list com)
  (* REPLACE THIS LINE WITH ":= _your_definition_ ." *).

Lemma iff_trans : forall (P1 P2 P3 : Prop), (P1 <-> P2) -> (P2 <-> P3) -> (P1 <-> P3).