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 Th20:
  {x,y} c= {z} implies x = z
proof
A1: x in {x,y} by TARSKI:def 2;
  assume {x,y} c= {z};
  then x in {z} by A1;
  hence thesis by TARSKI:def 1;
end;
