theorem Th4:
  x in union C implies {x} in C
proof
  assume x in union C;
  then consider X such that
A1: x in X and
A2: X in C by TARSKI:def 4;
  {x} c= X by A1,ZFMISC_1:31;
  hence thesis by A2,CLASSES1:def 1;
end;
