def __init__(self, groups=None):
        super(GroupedRequirements, self).__init__(NamedRequirements)
        self.groups = groups or self.default_groups
        self.filename = None
        self.create_default_groups()
def _get(delta, start):
            p = start
            while delta:
                p = p.next
                delta -= 1
            return p.val