from_document (Nokogiri::XML::RelaxNG)

    /*
     * call-seq:
     *  from_document(doc)
     *
     * Create a new RelaxNG schema from the Nokogiri::XML::Document +doc+
     */
    static VALUE from_document(VALUE klass, VALUE document)
    {
      xmlDocPtr doc;
      Data_Get_Struct(document, xmlDoc, doc);

      // In case someone passes us a node. ugh.
      doc = doc->doc;

      xmlRelaxNGParserCtxtPtr ctx = xmlRelaxNGNewDocParserCtxt(doc);

      VALUE errors = rb_ary_new();
      xmlSetStructuredErrorFunc((void *)errors, Nokogiri_error_array_pusher);

    #ifdef HAVE_XMLRELAXNGSETPARSERSTRUCTUREDERRORS
      xmlRelaxNGSetParserStructuredErrors(
        ctx,
        Nokogiri_error_array_pusher,
        (void *)errors
      );
    #endif

      xmlRelaxNGPtr schema = xmlRelaxNGParse(ctx);

      xmlSetStructuredErrorFunc(NULL, NULL);
      xmlRelaxNGFreeParserCtxt(ctx);

      if(NULL == schema) {
        xmlErrorPtr error = xmlGetLastError();
        if(error)
          Nokogiri_error_raise(NULL, error);
        else
          rb_raise(rb_eRuntimeError, "Could not parse document");

        return Qnil;
      }

      VALUE rb_schema = Data_Wrap_Struct(klass, 0, dealloc, schema);
      rb_iv_set(rb_schema, "@errors", errors);

      return rb_schema;
    }