code: def builtin(self, ref, context=None):
          
          w3 = 'http://www.w3.org'
          try:
              if isqref(ref):
                  ns = ref[1]
                  return ref[0] in Factory.tags and ns.startswith(w3)
              if context is None:
                  context = self.root
              prefix = splitPrefix(ref)[0]
              prefixes = context.findPrefixes(w3, 'startswith')
              return prefix in prefixes and ref[0] in Factory.tags
          except:
              return False
docs: Get whether the specified reference is an (xs) builtin.
          @param ref: A str or qref.
          @type ref: (str|qref)
          @return: True if builtin, else False.
          @rtype: bool