/** if this element always matches exactly one token, return a topology covering exactly those possible tokens, otherwise <tt>null</tt> */
abstract Topology toAtom();
/** if this element always matches exactly one token, return a topology covering exactly those possible tokens, otherwise <tt>null</tt> */
abstract Topology toAtom();
Forest epsilonForm() { throw new Error("no epsilon form: " + this); }
final boolean possiblyEpsilon(Walk.Cache cache) {
Boolean ret = cache==null ? null : cache.possiblyEpsilon.get(this);
Forest epsilonForm() { throw new Error("no epsilon form: " + this); }
final boolean possiblyEpsilon(Walk.Cache cache) {
Boolean ret = cache==null ? null : cache.possiblyEpsilon.get(this);