  When we have a morphism f : P^n -> P^n, then we have an inequality
  \frac{1}{\deg f} h(f(P)) +C > h(P) which provides a good upper bound of
$h(P)$. However, if $f$ is a rational map, then \frac{1}{\deg f} h(f(P))+C
cannot be an upper bound of h(P). In this paper, we will define the $D$-ratio
of a rational map $f$ which will replace the degree of a morphism in the height
inequality of h(P).
