Binomial coefficients for negative args

  (Would get bitten by this a few days ago — if I had been using 2.16.1.)

We teach small kids¹⁾ math — and when²⁾ we cover binomials, most of
the kids (even 1st-graders) understand why binomials are not
well-defined for negative arguments.

  ¹⁾ Grades 1–4 (ages 6–10).

  ²⁾ We invented topics covering about 6–7 years of teaching.  So we
     do not cover binomials every year.

While we do not cover meromorphic functions of several variables in
our program, it is a shame that mathematicians WHO SHOULD KNOW fall
down this (trivial!) trap…

Hope this helps,
Ilya