def filter_composite_from_subgroups(s):
    """
    Given a sorted list of subgroups, return a string appropriate to provide as
    the a composite track's `filterComposite` argument

    >>> import trackhub
    >>> trackhub.helpers.filter_composite_from_subgroups(['cell', 'ab', 'lab', 'knockdown'])
    'dimA dimB'

    Parameters
    ----------
    s : list
        A list representing the ordered subgroups, ideally the same list
        provided to `dimensions_from_subgroups`. The values are not actually
        used, just the number of items.
    """
    dims = []
    for letter, sg in zip('ABCDEFGHIJKLMNOPQRSTUVWZ', s[2:]):
        dims.append('dim{0}'.format(letter))
    if dims:
        return ' '.join(dims)