code: function choose (n, k) {
                      if(typeof n !== 'number' || typeof k !== 'number') return NaN;
                      if(k === 0) return 1;
                      return (n * choose(n - 1, k - 1)) / k;
                  }
docs: Recursively computes n choose k.
  @param {Number} n A number.
  @param {Number} k A number.
  @return {Number|Infinity} n choose k.