Filter {@link XMLGrammarPool} that exposes a read-only view of the underlying pool.
this.core = pool;
// noop. don't let caching to happen
// noop. cache is read-only.
// noop. this pool is always read-only
return core.retrieveGrammar(desc);
return core.retrieveInitialGrammarSet(grammarType);
// noop. this pool is always read-only.