∀ {ι : Type u_1} {R : Type u_2} {M : Type u_3} {N : Type u_4} [inst : CommRing R] [inst_1 : AddCommGroup M]
  [inst_2 : AddCommGroup N] [inst_3 : Module R M] [inst_4 : Module R N] (Q : QuadraticMap R M N) {g : ι → M}
  (l : ι →₀ R),
  Q ((Finsupp.linearCombination R g) l) =
    (Finsupp.linearCombination R (QuadraticMap.polarSym2 ⇑Q ∘ Sym2.map g)) l.sym2Mul -
      (Finsupp.linearCombination R (⇑Q ∘ g)) (l * l)