theorem Th16:
  {x,y} in C implies x in union C & y in union C
proof
A1: {x} c= {x,y} & {y} c= {x,y} by ZFMISC_1:7;
A2: x in {x} & y in {y} by TARSKI:def 1;
  assume {x,y} in C;
  hence thesis by A1,A2,TARSKI:def 4;
end;
