Remark subFormulaNTEHelp :
 forall (f g : Formula) (v : nat) (s : Term),
 SysPrf (Ensembles.Add _ (Empty_set Formula) f) g ->
 SysPrf (Ensembles.Add _ (Empty_set Formula) (substituteFormula L f v s))
   (substituteFormula L g v s).Remark subFormulaNTE :
 forall (f : Formula) (T : System),
 (forall (v : nat) (s : Term),
  ~ In v (freeVarFormula L f) ->
  SysPrf T (iffH (substituteFormula L f v s) f)) /\
 (forall (v1 v2 : nat) (s : Term),
  ~ In v2 (list_remove _ eq_nat_dec v1 (freeVarFormula L f)) ->
  SysPrf T
    (iffH (substituteFormula L (substituteFormula L f v1 (var v2)) v2 s)
       (substituteFormula L f v1 s))) /\
 (forall (v1 v2 : nat) (s1 s2 : Term),
  v1 <> v2 ->
  ~ In v2 (freeVarTerm L s1) ->
  ~ In v1 (freeVarTerm L s2) ->
  SysPrf T
    (iffH (substituteFormula L (substituteFormula L f v1 s1) v2 s2)
       (substituteFormula L (substituteFormula L f v2 s2) v1 s1))).

Lemma subFormulaExch : forall (f : Formula) (T : System) (v1 v2 : nat) (s1 s2 : Term), v1 <> v2 -> ~ In v2 (freeVarTerm L s1) -> ~ In v1 (freeVarTerm L s2) -> SysPrf T (iffH (substituteFormula L (substituteFormula L f v1 s1) v2 s2) (substituteFormula L (substituteFormula L f v2 s2) v1 s1)).