import itertools


def all_liouville_subspaces(hilbert_subspace):
    """
    Given a Hilbert subspace, return a comma separated list with all included
    Liouville subspaces

    Example
    -------
    >>> all_liouville_subspaces('ge')
    'gg,ge,eg,ee'
    """
    return ','.join(''.join(s) for s
                    in itertools.product(hilbert_subspace, repeat=2))