quad = function() {
    this._geom = _geom;
    this._tree = null;
    this._cache = {
      query: false,
      result: false
    };
  }
The quad core that will become the sigma interface with the quadtree.

property {object} _tree  Property holding the quadtree object.
property {object} _geom  Exposition of the _geom namespace for testing.
property {object} _cache Cache for the area method.