constant p : nat → Prop
open tactic
set_option pp.all true

definition ex (a b c : nat) (H : p c) : a = b → p a → p b :=
by do intro `H1, intro `H2,
      get_local `a >>= subst,
      trace_state,
      assumption

#print ex

example (a b c : nat) (H : p c) : a = b → p a → p b :=
by do intros,
      get_local `b >>= subst,
      trace_state,
      assumption