Over-/Underbrace

`\underbrace{a+\overbrace{b+ \cdots + y}^{123}+z}_{\alpha\beta\gamma}`
`\overbrace{a+\underbrace{b+c}+d}`
`V_n^m=\prod_{i=0}^{m-1}(n-i)= \underbrace{n(n-1)(n-2)\cdots (n-m+1)}_{m\;\mbox{factors}} =\frac{n!}{(n-m)!}`

® CS 1998, 2003