Two new families of languages, the F(k) and U(k) languages, are Introduced each of which is, in some sense, a generalization of top-down deterministic languages. This leads us to new characterizations of s-grammars and LL(1) languages. We include a characterization of the unambiguous context-free languages, generalizations of the equivalence relation on s-grammars to LL(k) grammars, and a summa...

We introduce the notion of discrimination as a generalization of both sorting and partitioning and show that discriminators (discrimination functions) can be defined generically, by structural recursion on order and equivalence expressions denoting a rich class of total preorders and equivalence relations, respectively. Discriminators improve the asymptotic performance of generic comparison-bas...

Within African savannas, elephants often damage individual trees to the extent that they influence tree density. New research shows that mutualistic ants inhabiting certain species of Acacia protect trees from catastrophic herbivory by elephants. Protection by the ants stabilizes tree cover across savannas in what is otherwise a highly dynamic biome.