  A left-corner parsing algorithm with top-down filtering has been reported to
show very efficient performance for unification-based systems. However, due to
the nontermination of parsing with left-recursive grammars, top-down
constraints must be weakened. In this paper, a general method of maximizing
top-down constraints is proposed. The method provides a procedure to
dynamically compute *restrictor*, a minimum set of features involved in an
infinite loop for every propagation path; thus top-down constraints are
maximally propagated.
