In computer science, one Counter automaton 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:
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "http://localhost:6011/en.wikipedia.org/v1/":): {\displaystyle \{ xy \in\ {a, b}{*} : numberofa(x) = numberofb(y)} \} }
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.