open tactic

definition tst1 (a : nat) : a = a :=
by do
   assert "x" (expr.const "nat" []),
   trace_state,
   a ← get_local "a",
   exact a,
   x ← get_local "x",
   mk_app ("eq" <.> "refl") [x] >>= exact

print tst1

definition tst2 (a : nat) : a = a :=
by do
   assert "x" (expr.const "nat" []),
   a ← get_local "a",
   exact a,
   trace "------------",
   trace_state,
   revert "x",
   intro "y",
   trace_state,
   y ← get_local "y",
   mk_app ("eq" <.> "refl") [y] >>= exact

print tst2