Create a short, informative, but not-so-unique identifying string for a trained model
    If a str suffix is provided then just pass it through.
    >>> compose_suffix(num_docs=100, num_topics=20)
    '_100X20'
    >>> compose_suffix(suffix='_sfx')
    '_sfx'
    >>> compose_suffix(suffix='')
    ''
    >>> compose_suffix(suffix=None)
    '_0X0'

def compose_suffix(num_docs=0, num_topics=0, suffix=None):
    """Create a short, informative, but not-so-unique identifying string for a trained model
    If a str suffix is provided then just pass it through.
    >>> compose_suffix(num_docs=100, num_topics=20)
    '_100X20'
    >>> compose_suffix(suffix='_sfx')
    '_sfx'
    >>> compose_suffix(suffix='')
    ''
    >>> compose_suffix(suffix=None)
    '_0X0'
    """
    if not isinstance(suffix, basestring):
        suffix = '_{}X{}'.format(num_docs, num_topics)
    return suffix