theorem Th60: union X c= Y implies X c= bool Y
proof
assume A1: union X c= Y & not X c= bool Y;
  then consider A being object such that
A2: A in X & not A in bool Y;
  reconsider AA=A as set by TARSKI:1;
AA c= Y by A2, A1, SETFAM_1:41; hence contradiction by A2;
end;
