theorem
  q => (p 'or' q) is tautology
  proof
    let M;
    thus (SAT M).(q => (p 'or' q)) = (SAT M).q => (SAT M).(p 'or' q) by Def11
    .= (SAT M).q => ((SAT M).p 'or' (SAT M).q) by semdis2
    .= 1 by XBOOLEAN:123;
  end;
