show_tac.lean:6:29: error: don't know how to synthesize placeholder
a b : Prop,
Hab : a ∧ b,
Ha : a,
Hb : b
⊢ b
show_tac.lean:6:2: error:invalid 'exact' tactic, term still contains metavariables after elaboration
  show b ∧ a, from and.intro ?M_1 Ha
proof state:
a b : Prop,
Hab : a ∧ b,
Ha : a,
Hb : b
⊢ b ∧ a
show_tac.lean:7:0: error: failed to add declaration 'example' to environment, value has metavariables
remark: set 'formatter.hide_full_terms' to false to see the complete term
  λ a b Hab,
    ?M_1