'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'

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
