show from_prod (q * n_k + p * m_j, q * m_j + p * n_k)
   ≃ from_prod ((qn + pm) + (qk + pj), (qm + pn) + (qj + pk))
ℕ : Type
inst✝ : Natural ℕ
q p n m k j : ℕ
n_k : ℕ := n + k
m_j : ℕ := m + j
qn : ℕ := q * n
qm : ℕ := q * m
qk : ℕ := q * k
qj : ℕ := q * j
pn : ℕ := p * n
pm : ℕ := p * m
pk : ℕ := p * k
pj : ℕ := p * j
⊢ (q * n_k + p * m_j)——(q * m_j + p * n_k) ≃ (qn + pm + (qk + pj))——(qm + pn + (qj + pk))
ℕ : Type
inst✝ : Natural ℕ
q p n m k j : ℕ
n_k : ℕ := n + k
m_j : ℕ := m + j
qn : ℕ := q * n
qm : ℕ := q * m
qk : ℕ := q * k
qj : ℕ := q * j
pn : ℕ := p * n
pm : ℕ := p * m
pk : ℕ := p * k
pj : ℕ := p * j
⊢ from_prod (q * n_k + p * m_j, q * m_j + p * n_k) ≃ from_prod (qn + pm + (qk + pj), qm + pn + (qj + pk))