/** map from StringPair(namespace,label) to ExternalElementExp. */
private final Map externalElementExps = new java.util.HashMap();
private ExternalElementExp getExtElementExp( String namespace, String label )
{
StringPair name = new StringPair(namespace,label);
ExternalElementExp exp = (ExternalElementExp)externalElementExps.get(name);
if( exp!=null ) return exp;
exp = new ExternalElementExp( pool, namespace, label, new LocatorImpl(getLocator()) );
externalElementExps.put( name, exp );
return exp;
}