code: protected final String findElemPrefix(String nsURI, SimpleOutputElement elem)
          throws XMLStreamException
      {
          /* Special case: empty NS URI can only be bound to the empty
           * prefix...
           */
          if (nsURI == null || nsURI.length() == 0) {
              String currDefNsURI = elem.getDefaultNsUri();
              if (currDefNsURI != null && currDefNsURI.length() > 0) {
                  // Nope; won't do... has to be re-bound, but not here:
                  return null;
              }
              return "";
          }
          return mCurrElem.getPrefix(nsURI);
      }
docs: Method called to find an existing prefix for the given namespace,
  if any exists in the scope. If one is found, it's returned (including
  "" for the current default namespace); if not, null is returned.
  
  @param nsURI URI of namespace for which we need a prefix