'Get whether to skip this I{content}.
Should be skipped when the content is optional
and either the value=None or the value is an empty list.
@param content: The content to skip.
@type content: L{Object}
@return: True if content is to be skipped.
@rtype: bool'

def skip(self, content):

    if self.optional(content):
        v = content.value
        if (v is None):
            return True
        if (isinstance(v, (list, tuple)) and (len(v) == 0)):
            return True
    return False
