[Rd] choose incorrect for fractional and some negative integer values (PR#10766)

[jerry.lewis at biogenidec.com]

choose(-5,-7) uses integer arguments (as specified in Help)  and returns a 
numeric value that is incorrect.  Either the function or the documentation 
should be fixed.  If the function is not fixed, a warning or an error 
would be helpful.

The fact that choose(n,k) usually returns choose(n,round(k,0)) is not 
obvious from either the output or the documentation.  I suggest issuing a 
warning when k is not an integer; a user might easily expect that choose 
would coerce both arguments to integers, but only coercing k to an integer 
is unusual and potentially misleading.

The fact that choose(n,k) always rounds .5 up in k (contrary to the round 
function documentation) is nowhere documented.

Help for coerce would lead a reader to expect k to be truncated toward 
zero (as.integer), instead of rounding it.

Jerry
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