pushdown的用法和样例：
词汇搭配
- weighted pushdown language 加权叠加语言...
- two-way pushdown automaton 双向下推自动机...
- pushdown stack 下推堆
- primitive pushdown machine 原始下推机
- pushdown storage 下推存储器
- stochastic pushdown language 随机下推语言...
- deterministic pushdown automaton 确定性下推自动机...
- associative pushdown memory 相联存储器, 相联叠...
- stochastic pushdown automata 随机下推自动机...
- pushdown list 下推表
- pushdown dialing 按压式拨号
- push down 下推
