theorem Th10:
  E (\/) T in Bool M
proof
  E c= M & T c= M by PBOOLE:def 18;
  then E (\/) T c= M (\/) M by PBOOLE:20;
  then E (\/) T is ManySortedSubset of M by PBOOLE:def 18;
  hence thesis by Def1;
end;
