theorem
  Free(p <=> q) = Free p \/ Free q
proof
  p <=> q = (p => q) '&' (q => p) by QC_LANG2:def 4;
  hence Free(p <=> q) = Free (p => q) \/ Free (q => p) by Th42
    .= Free p \/ Free q \/ Free (q => p) by Th60
    .= Free p \/ Free q \/ (Free p \/ Free q) by Th60
    .= Free p \/ Free q;
end;
