Section LNT_FixPoint.
Let codeFormula := codeFormula LNT codeLNTFunction codeLNTRelation.

Lemma FixPointLNT :
forall (A : Formula) (v : nat),
{B : Formula |
SysPrf PA
(iffH B (substituteFormula LNT A v (natToTermLNT (codeFormula B)))) /\
(forall x : nat,
In x (freeVarFormula LNT B) <->
In x (list_remove _ eq_nat_dec v (freeVarFormula LNT A)))}.