code: function polygonCanSee2(polygon, a,b) {
      // for each edge
      for (var i = 0; i !== polygon.length; ++i) {
          // ignore incident edges
          if (i === a || i === b || (i + 1) % polygon.length === a || (i + 1) % polygon.length === b){
              continue;
          }
          if( lineSegmentsIntersect(polygonAt(polygon, a), polygonAt(polygon, b), polygonAt(polygon, i), polygonAt(polygon, i+1)) ){
              return false;
          }
      }
      return true;
  }
docs: Check if two vertices in the polygon can see each other
  @method canSee2
  @param  {Number} a Vertex index 1
  @param  {Number} b Vertex index 2
  @return {Boolean}