  We present a uniform approach for solving language inclusion problems. Our
approach relies on a least fixpoint characterization and a quasiorder to
compare words of the "smaller" language, reducing the inclusion check to a
finite number of membership queries in the "larger" language. We present our
approach in detail on the case of inclusion of a context-free language given by
a grammar into a regular language. We then explore other inclusion problems and
discuss how to apply our approach.
