'Args:
lmdb_data: a :class:`LMDBData` instance.
decoder (k,v -> dp | None): a function taking k, v and returning a datapoint,
or return None to discard.'

def __init__(self, lmdb_data, decoder):

    def f(dp):
        return decoder(dp[0], dp[1])
    super(LMDBDataDecoder, self).__init__(lmdb_data, f)
