Then n!/l! gives the usual results for n!, including 0! = 1 and n! = oo for n < 0. Now, however,
when n and r are both negative, instead of the indeterminate oo/co, a specific value is defined.
The product of two factorial ratios [n!/r!][m!/s!] is the indeterminate 0 .oo, precisely when
n >_ 0 > r and m < 0 < s, since then