∀ {L : FirstOrder.Language} {α : Type u'} {β : Type v'} {γ : Type u_1} (t : L.Term ((γ ⊕ α) ⊕ β)),
  FirstOrder.Language.Term.constantsVarsEquivLeft.symm t =
    (FirstOrder.Language.Term.relabel (⇑(Equiv.sumAssoc γ α β)) t).varsToConstants