Notation Formula := (Formula L) (only parsing).Notation Formulas := (Formulas L) (only parsing).Notation System := (System L) (only parsing).Notation Term := (Term L) (only parsing).Notation Terms := (Terms L) (only parsing).

Lemma eqSym : forall (T : fol.System L) (a b : fol.Term L), SysPrf T (equal a b) -> SysPrf T (equal b a).