theorem
  {x1,x2} c= {y1,y2} implies x1 = y1 or x1 = y2
proof
  assume {x1,x2} c= {y1,y2};
  then {x1,x2}={} or {x1,x2}={y1} or {x1,x2}={y2} or {x1,x2}={y1,y2} by Lm13;
  hence thesis by Th4,Th6;
end;
