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

theorem Th11:
  for r,s being Element of CQC-WFF(A) holds
  r 'or' s is Element of CQC-WFF(A)
proof
  let r,s be Element of CQC-WFF(A);
  r 'or' s = 'not' ('not' r '&' 'not' s) by QC_LANG2:def 3;
  hence thesis;
end;
