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 dfs(self,candidates,target,start,reslist):
        length = len(candidates)
        if target == 0: 
            return self.result.append(reslist)