reserve A for QC-alphabet;
reserve i,j,k for Nat;
reserve f for Substitution of A;

theorem Th6:
  VERUM(A) is Element of CQC-WFF(A)
proof
    Fixed VERUM(A) = {} & Free VERUM(A) = {} by QC_LANG3:53,63;
    hence thesis by Th4;
end;
