code: function FPNode(item, parent) {
                      if (item === void 0) { item = null; }
                      if (parent === void 0) { parent = null; }
                      this.item = item;
                      this.parent = parent;
                      /**
                       * Support of the FPNode. (a.k.a. "count" as defined by Han).
                       */
                      this.support = 1;
                      /**
                       * nextSameItemNode (a.k.a. "Node-link" as defined by Han):
                       * Links to the next node in the FP-tree carrying the same
                       * item, or null if there is none.
                       */
                      this.nextSameItemNode = null;
                      /**
                       * PUBLIC READONLY. Children of the FPNode in an array. Empty array if there is none.
                       */
                      this._children = [];
                  }
docs: FPNode composes an FPTree and represents a given item a item-prefix subtree.
  It keeps track of its parent if it has any, and lists his children FPNodes.
  
  @param  {T}         item   The item it represents.
  @param  {FPNode<T>} parent His parent, if it has any.