theorem
  Fixed(p <=> q) = Fixed p \/ Fixed q
proof
  p <=> q = (p => q) '&' (q => p) by QC_LANG2:def 4;
  hence Fixed(p <=> q) = Fixed (p => q) \/ Fixed (q => p) by Th42
    .= Fixed p \/ Fixed q \/ Fixed (q => p) by Th70
    .= Fixed p \/ Fixed q \/ (Fixed q \/ Fixed p) by Th70
    .= Fixed p \/ Fixed q;
end;
