code: function sinval(a, b, c) {
  			return sinsign(a, b, c) / dist(a, b) / dist(b, c);
  		}
docs: /* js/src/d2/irrational/sinval.js 
  Typically this kind of computation is not allowed. Computing distances
  between two vertices in the general case requires to compute the square root
  of a number. We only work with rationals in our algorithms and cannot handle
  irrationals that could appear when allowing the use of square roots.