Pushdown automaton configuration
= Pushdown automaton configuration
{title2=$(q,w,\alpha)$}
The control state $q$, unread input <word> $w$, and <stack> contents $\alpha$ of a <pushdown automaton>. A fixed convention must say which end of $\alpha$ is the top. With the top on the left, replacing top $X$ by $\gamma$ changes $X\beta$ into $\gamma\beta$.