theorem Th70:
  Fixed(p => q) = Fixed p \/ Fixed q
proof
  p => q = 'not'(p '&' 'not' q) by QC_LANG2:def 2;
  hence Fixed(p => q) = Fixed(p '&' 'not' q) by Th39
    .= Fixed p \/ Fixed 'not' q by Th42
    .= Fixed p \/ Fixed q by Th39;
end;
