Collect "extra == ..." operands from a marker.

    Returns a list of str. Each str is a speficied extra in this marker.

def get_contained_extras(marker):
    """Collect "extra == ..." operands from a marker.

    Returns a list of str. Each str is a speficied extra in this marker.
    """
    if not marker:
        return set()
    marker = Marker(str(marker))
    extras = set()
    _markers_collect_extras(marker._markers, extras)
    return extras