reserve A for QC-alphabet;
reserve p, q, r, s for Element of CQC-WFF(A);

theorem Th20:
  ( p '&' q ) => ( p 'or' q ) in TAUT(A)
proof
  p => ( p 'or' q ) in TAUT(A) & ( p '&' q ) => p in TAUT(A) by Th3,Th19;
  hence thesis by LUKASI_1:3;
end;
