How many ways are there of filling the bin. The ways involving non-bin
parts of the histogram are a constant factor.
The bin is filled by a selection from the sampling events. Any five of
the sampcount events may be involved in filling the bin. The sampcount
events happen always in the same order, so I don't think we want
permutations of them ... I'm guessing that this is a combinations
situation. Where using the concept of combinations will give the
proportional number of samplings that fill the bin to that height.