theorem Th15:
  p => (q => p) is tautology
  proof
    let M;
    thus (SAT M).(p => (q => p)) = (SAT M).p => (SAT M).(q => p) by Def11
   .= (SAT M).p => ((SAT M).q => (SAT M).p) by Def11
   .= 1 by XBOOLEAN:104;
  end;
