public Set<String> intersection(AttributeDict other) {
        if ( other==null || other.size()==0 || size()==0 ) {
            return Collections.emptySet();
        }

		Set<String> result = new HashSet<String>(attributes.keySet());
		result.retainAll(other.attributes.keySet());
		return result;
    }
Return the set of keys that collide from
 {@code this} and {@code other}.
tool/src/org/antlr/v4/tool/AttributeDict.java