def parse_systemtype(system_spec):
    """
    Parse a system type/summary specification.

    Examples:
    - "4att3ms" to match four-attractor systems with three species concentrations' monotonically correlated to each other
    - "4" to match four-attractor systems
    - "else" for a default rule

    Returns an object usable as a key by specificrulevalue.
    """
    if system_spec == 'else':
        return None
    elif 'att' in system_spec:
        att, ms_rest = system_spec.split('att')
        return (int(att), int(ms_rest.split('ms')[0]))
    else:
        return int(system_spec)