Constructive Learning of Context-Free Languages with a Subpansive Tree

. A subpansive tree is a rooted tree that gives a partial order of nonterminal symbols of a context-free grammar. We formalize subpansive trees as background knowledge of CFGs, and investigate query learning of CFGs with the help of subpansive trees. We show a restricted class of CFGs, which we call hierarchical CFGs, is e#ciently learnable, while it is unlikely to be polynomial-time predictable. 1 Introduction Language acquisition is one of the central interests to both theoretical computer science and linguistics. In computational learning theory, Angluin [2] showed that the regular languages are e#ciently learnable, proposing a polynomial-time algorithm on finite automata with terminal membership queries and terminal equivalence queries. Since then, the learnability of context-free classes has come to be the next research topic, and a number of studies have been reported under di#erent acquisition scenarios: Angluin [1] allowed nonterminal membership queries for learning a k-bound..