  Controlled natural languages (CNL) with a direct mapping to formal logic have
been proposed to improve the usability of knowledge representation systems,
query interfaces, and formal specifications. Predictive editors are a popular
approach to solve the problem that CNLs are easy to read but hard to write.
Such predictive editors need to be able to "look ahead" in order to show all
possible continuations of a given unfinished sentence. Such lookahead features,
however, are difficult to implement in a satisfying way with existing grammar
frameworks, especially if the CNL supports complex nonlocal structures such as
anaphoric references. Here, methods and algorithms are presented for a new
grammar notation called Codeco, which is specifically designed for controlled
natural languages and predictive editors. A parsing approach for Codeco based
on an extended chart parsing algorithm is presented. A large subset of Attempto
Controlled English (ACE) has been represented in Codeco. Evaluation of this
grammar and the parser implementation shows that the approach is practical,
adequate and efficient.
