  This paper presents a restricted form of linear indexed grammars, called even
linear indexed grammars, which yield the even linear indexed languages. These
languages properly contain the context-free languages and are contained in the
set of linear indexed languages. We show that several patterns found in natural
languages are also generated by these grammars, including crossing
dependencies, copying, and multiple agreements. We discuss the learning problem
for even linear indexed languages and show that it is reducible to that of the
context-free languages. The closure properties for this class of languages are
also presented.
