code: public Source resolveURI(
            String base, String urlString, SourceLocator locator)
              throws TransformerException, IOException
    {
  
      Source source = null;
  
      if (null != m_uriResolver)
      {
        source = m_uriResolver.resolve(urlString, base);
      }
  
      if (null == source)
      {
        String uri = SystemIDResolver.getAbsoluteURI(urlString, base);
  
        source = new StreamSource(uri);
      }
  
      return source;
    }
docs: This will be called by the processor when it encounters
  an xsl:include, xsl:import, or document() function.
  
  @param base The base URI that should be used.
  @param urlString Value from an xsl:import or xsl:include's href attribute,
  or a URI specified in the document() function.
  
  @return a Source that can be used to process the resource.
  
  @throws IOException
  @throws TransformerException