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

theorem Th14:
  for x being bound_QC-variable of A,r being Element of CQC-WFF(A) holds
  Ex(x,r) is Element of CQC-WFF(A)
proof
  let x be bound_QC-variable of A,r be Element of CQC-WFF(A);
  Ex(x,r) = 'not' All(x,'not' r) by QC_LANG2:def 5;
  hence thesis;
end;
