code: public String getNamespaceURIFromPrefix(String prefix)
      {
          String uri = null;
          if (m_prefixMap != null)
              uri = m_prefixMap.lookupNamespace(prefix);
          return uri;
      }
docs: Returns the URI of prefix (if any)
  
  @param prefix the prefix whose URI is searched for
  @return the namespace URI currently associated with the
  prefix, null if the prefix is undefined.