571.lean:6:2: error:invalid 'cases' tactic, 'Exists' can only eliminate to Prop
proof state:
P : ℕ → Prop,
H : ∃ (n : ℕ), P n
⊢ ℕ
571.lean:7:0: error: don't know how to synthesize placeholder
P : ℕ → Prop,
H : ∃ (n : ℕ), P n
⊢ ℕ
571.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 (P : …) (H : …),
    ?M_1