/** add all positions reachable from the start of this Element to @rp */
abstract void reachable(HashSet<Sequence.Position> rp);
+ 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);