∀ {α : Type u_1} {M : Type u_2} [inst : Zero M] (f : α →₀ M) (a : α) (b : M) [inst_1 : DecidableEq α]
  [inst_2 : DecidableEq M], (f.update a b).support = if b = 0 then f.support.erase a else insert a f.support