The equivalence problem for unambiguous grammars is an important, but very difficult open question in formal language theory. Consider the \emph{limited} equivalence problem for unambiguous grammars -- for two unambiguous grammars $G_1$ and $G_2$, tell whether or not they describe the same set of words of length $n$. Obviously, the naive approach requires exponential time with respect to $n$. By combining two classic algorithmic ideas, I introduce a $O({\rm poly}(n, |G_1|, |G_2|))$ algorithm for this problem. Moreover, the ideas behind the algorithm prove useful in various other scenarious.