code: String getPrefix (String uri)
      {
          if (uriTable == null) {
              return null;
          } else {
              return (String)uriTable.get(uri);
          }
      }
docs: Look up one of the prefixes associated with a URI in this context.
  
  <p>Since many prefixes may be mapped to the same URI,
  the return value may be unreliable.</p>
  
  @param uri The URI to look up.
  @return The associated prefix, or null if none is declared.
  @see org.xml.sax.helpers.NamespaceSupport2#getPrefix