Setlist Rareness

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Setlist Rareness for a show is computed as follows.

* CNT(song) = total number of times a song was played on all other shows
* noShows = number of shows so far
* P(song) = CNT(song) / (noShows-1) = probability that a song appears in a setlist of another show
* P_avg(setlist) = sum_i [ P(song_i) ] / length(setlist)
* Setlist Rareness = 1 - P_avg

Setlist rareness is always a number in range [0,1]:
- 0 = most ordinary setlist possible (all songs in setlist were played at all other shows)
- 1 = rarest setlist possible (none of the songs in the setlist was played at any of the other shows)

Please note that these rareness numbers are relative and do no make much sense on their own,
if they make sense at all ...