exact_partial.lean:4:2: error:invalid 'exact' tactic, term still contains metavariables after elaboration
  and.intro ?M_1 ?M_2
proof state:
a b : Prop,
a_1 : a,
a_2 : b
⊢ a ∧ b
exact_partial.lean:5:0: error: don't know how to synthesize placeholder
a b : Prop
⊢ a → b → a ∧ b
exact_partial.lean:5:0: error: failed to add declaration '14.0' to environment, value has metavariables
remark: set 'formatter.hide_full_terms' to false to see the complete term
  λ (a b : Prop),
    ?M_1