description:
spick-elt seq
randomly pick a superposition from the given sequence, with equal probability
examples:
spick-elt (|a> + |b> + |c> . |d> . |e> + |f> . |g> + |h> + |i> + |j>)
|e> + |f>
see also:
weighted-pick-elt, pick-elt, sweighted-pick-elt