'Combine multiple sets of recs, preserving order.
Picks items equally from each input sr, which can be useful for
getting a diverse set of recommendations instead of one that matches
a theme. Preserves ordering, so all rank 1 recs will be listed first,
then all rank 2, etc.
Returns a list of id36s.'

@classmethod
def _merge_roundrobin(cls, rows):

    return roundrobin(*[row._values().itervalues() for row in rows])
