p ← s ##.birthday n                         ⍝ Probability of same birthday.

"In  a  group  of 23 people there is a better than 50-50 chance that two of them
 share a birthday".

This function from Nicolas Delcros and Roger Hui  generalises  this assertion to
find the probability of a pair of coincident results  occuring  from  n  trials,
each with probability ÷s.

See: http://www.jsoftware.com/jwiki/Essays/Birthday_Problem

Examples:

    365 birthday 23       ⍝ probability that 2 people out of 23 share a birthday
0.5072972343

    6 birthday 2          ⍝ probability of a pair in dice-throwing.
0.1666666667

    (2*32) birthday 1e4   ⍝ probability of BuildID collision for 10.000 builds
0.01157288999

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