A further note on top-down deterministic languages
D. Wood · The Computer Journal · 1971
Two new families of languages, the F(k) and U(k) languages, are introduced each of which is, in some sense, a generalisation of top-down deterministic languages. This leads us to new characterisations of s-languages and LL(1) languages. We include a characterisation of the unambiguous context-free languages, generalisations of the equivalence relation on s-grammars to s-separable sets, a summary of the non-closure results for LL(k), F(k) and U(k) languages, and it is shown that non-degenerate hierarchies exist for the families of F(k) and U(k) languages.