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 Th11:
  x <> y implies {x} misses {y}
proof
  assume x <> y;
  then not x in {y} by TARSKI:def 1;
  hence thesis by Lm6;
end;
