code: @InterfaceAudience.Private
      public static boolean isValidDocumentId(String id) {
          // http://wiki.apache.org/couchdb/HTTP_Document_API#Documents
          if (id == null || id.length() == 0)
              return false;
          if (id.charAt(0) == '_')
              return (id.startsWith("_design/"));
          return true;
          // "_local/*" is not a valid document ID.
          // Local docs have their own API and shouldn't get here.
      }
docs: in CBLDatabase+Insertion.m
  + (BOOL) isValidDocumentID: (NSString*)str