In computer science, one ♠ is a Pushdown automaton with only two symbols in (A and the initial symbol) (the finite set of stack symbols), this automaton can be used for accept one subset of Context free languages, example the language:
For accept the previous language, the automaton can use the symbol A for count the number of a in x (writing A per each A in x) and deleting one A per each b in y.
Accepted languages of counter automaton are regular languages and some context-free languages.