reserve u,v,x,x1,x2,y,y1,y2,z,p,a for object,
        A,B,X,X1,X2,X3,X4,Y,Y1,Y2,Z,N,M for set;

theorem Th3:
  {x} c= {y} implies x = y
proof
  x in { x } by TARSKI:def 1;
  then {x} = {y} implies x = y by TARSKI:def 1;
  hence thesis by Lm3;
end;
