code: function _quadIndex(point, quadBounds) {
      var xmp = quadBounds.x + quadBounds.width / 2,
          ymp = quadBounds.y + quadBounds.height / 2,
          top = (point.y < ymp),
          left = (point.x < xmp);
  
      if (top) {
        if (left)
          return 0;
        else
          return 1;
      }
      else {
        if (left)
          return 2;
        else
          return 3;
      }
    }
docs: Quad Functions
  ------------
  
  The Quadtree functions themselves.
  For each of those functions, we consider that in a splitted quad, the
  index of each node is the following:
  0: top left
  1: top right
  2: bottom left
  3: bottom right
  
  Moreover, the hereafter quad's philosophy is to consider that if an element
  collides with more than one nodes, this element belongs to each of the
  nodes it collides with where other would let it lie on a higher node.
   
  Get the index of the node containing the point in the quad
  
  @param  {object}  point      A point defined by coordinates (x, y).
  @param  {object}  quadBounds Boundaries of the quad (x, y, width, heigth).
  @return {integer}            The index of the node containing the point.