  The pumping lemma for context-free languages is a result about pushdown
automata which is strikingly similar to the well-known pumping lemma for
regular languages. However, though the lemma for regular languages is simply
proved by using the pigeonhole principle on deterministic automata, the lemma
for pushdown automata is proven through an equivalence with context-free
languages and through the more powerful Ogden's lemma. We present here a proof
of the pumping lemma for context-free languages which relies on pushdown
automata instead of context-free grammars.
