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 Th7:
  {x} c= {x,y}
proof
  let z;
  assume z in {x};
  then z=x by TARSKI:def 1;
  hence thesis by TARSKI:def 2;
end;
