theorem
  Fixed(p 'or' q) = Fixed p \/ Fixed q
proof
  p 'or' q = 'not'('not' p '&' 'not' q) by QC_LANG2:def 3;
  hence Fixed(p 'or' q) = Fixed('not' p '&' 'not' q) by Th39
    .= Fixed 'not' p \/ Fixed 'not' q by Th42
    .= Fixed p \/ Fixed 'not' q by Th39
    .= Fixed p \/ Fixed q by Th39;
end;
