@code{n} and @code{k} must be small integers >= 0. This function returns the
binomial coefficient
@tex
-${n \choose k} = {n! \over n! (n-k)!}$
+${n \choose k} = {n! \over k! (n-k)!}$
@end tex
@ifinfo
(@code{n} choose @code{k}) = @code{n}! / @code{k}! @code{(n-k)}!