'Create a recordset instance.
:param ids: a tuple of record ids
:param env: an environment
:param prefetch: an optional prefetch object'

@classmethod
def _browse(cls, ids, env, prefetch=None):

    records = object.__new__(cls)
    records.env = env
    records._ids = ids
    if (prefetch is None):
        prefetch = defaultdict(set)
    records._prefetch = prefetch
    prefetch[cls._name].update(ids)
    return records
