reserve u,v,x,y,z,X,Y for set;
reserve r,s for Real;

theorem Th3:
  <%u,v%> = <%x,y%> implies u = x & v = y
  proof
    <%u,v%>.0 = u & <%u,v%>.1 = v & <%x,y%>.0 = x & <%x,y%>.1 = y;
    hence thesis;
  end;
