code: function d3_geom_hullCCW(i1, i2, i3, v) {
    var t, a, b, c, d, e, f;
    t = v[i1]; a = t[0]; b = t[1];
    t = v[i2]; c = t[0]; d = t[1];
    t = v[i3]; e = t[0]; f = t[1];
    return ((f-b)*(c-a) - (d-b)*(e-a)) > 0;
  }
docs: are three points in counter-clockwise order?