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;
}
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}