Counter automaton

In computer science, a counter automaton is a Pushdown automaton with only two symbols, A and the initial symbol in $\Gamma\,$ (the finite set of stack symbols). This class of automata can recognize a subset of Context free languages, for instance the language:

$\{\ a^nb^n : n \in \mathbb{N} \}$

To accept the above language, let x be a word on the form above. The automaton can use the symbol A to count the number of a's in x (writing an A for each A in x) and deleting an A for each b in x.