code: function _walk(node){
  			var i, l, child, s, state, allSelected,someSelected,
  				children = node.children;
  
  			if( children && children.length ){
  				// check all children recursively
  				allSelected = true;
  				someSelected = false;
  
  				for( i=0, l=children.length; i<l; i++ ){
  					child = children[i];
  					// the selection state of a node is not relevant; we need the end-nodes
  					s = _walk(child);
  					if( s !== false ) {
  						someSelected = true;
  					}
  					if( s !== true ) {
  						allSelected = false;
  					}
  				}
  				state = allSelected ? true : (someSelected ? undefined : false);
  			}else{
  				// This is an end-node: simply report the status
  //				state = ( node.unselectable ) ? undefined : !!node.selected;
  				state = !!node.selected;
  			}
  			node._changeSelectStatusAttrs(state);
  			return state;
  		}
docs: Visit all end nodes and adjust their parent's `selected` and `partsel` attributes. Return selection state true, false, or undefined.