rcases is_fn with ⟨rel_sub, rel_uniq⟩
is_fn : (∀ x ∈ {p | p.1 + 3 * p.2 = 4}, x ∈ Set.univ ×ˢ Set.univ) ∧ ∀ (x : ℤ), ∃! y, (x, y) ∈ {p | p.1 + 3 * p.2 = 4}
⊢ False
case intro
rel_sub : ∀ x ∈ {p | p.1 + 3 * p.2 = 4}, x ∈ Set.univ ×ˢ Set.univ
rel_uniq : ∀ (x : ℤ), ∃! y, (x, y) ∈ {p | p.1 + 3 * p.2 = 4}
⊢ False