Discussion on answer by dan: Using 有 vs 是 to describe an object

A pushdown automata (PDA) is a machine that has finitely many states and a number of stacks. Each stack is a list of symbols, and we think of one end as the top and the other end as the bottom. At each step, the PDA does something and then transitions to another state, where its action and state transition depends only on its state and what is at the top of its stacks, and it is only allowed to remove or add symbols one at a time to the top of each stack.