'Refs present in this container under a base.
:param base: The base to return refs under.
:return: A set of valid refs in this container under the base; the base
prefix is stripped from the ref names returned.'

def subkeys(self, base):

    keys = set()
    base_len = (len(base) + 1)
    for refname in self.allkeys():
        if refname.startswith(base):
            keys.add(refname[base_len:])
    return keys
