10. Simplify: $\frac{m_{2}+m_{3}+\cdots+m_{n}}{m_{1}\left(m_{1}+m_{2}+\cdots+m_{n}\right)}-\frac{m_{2}}{m_{1}\left(m_{1}+m_{2}\right)}-\frac{m_{3}}{\left(m_{1}+m_{2}\right)\left(m_{1}+m_{2}+m_{3}\right)}-\cdots$
$$
-\frac{m_{n}}{\left(m_{1}+m_{2}+\cdots+m_{n-1}\right)\left(m_{1}+m_{2}+\cdots+m_{n}\right)}=\square .
$$
$0$