/** the root superclass for all components of the grammar (terminals, nonterminals, literals, etc) */
public abstract class Element {
+ /** if this element always matches exactly one token, return a topology covering exactly those possible tokens, otherwise <tt>null</tt> */
abstract Topology toAtom();
- public Topology noFollow() { return null; }
+
Forest epsilonForm() { throw new Error("no epsilon form: " + this); }
- final boolean possiblyEpsilon(Walk.Cache cache) {
+ final boolean possiblyEpsilon(Walk.Cache cache) {
Boolean ret = cache==null ? null : cache.possiblyEpsilon.get(this);
if (ret != null) return ret.booleanValue();
ret = new Walk.PossiblyEpsilon().walk(this) ? Boolean.TRUE : Boolean.FALSE;