code: function _walk(node) {
  				var i,
  					l,
  					child,
  					s,
  					state,
  					allSelected,
  					someSelected,
  					unselIgnore,
  					unselState,
  					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( !child.unselectableIgnore ) {
  						unselIgnore = FT.evalOption(
  							"unselectableIgnore",
  							child,
  							child,
  							opts,
  							false
  						);
  						if (!unselIgnore) {
  							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
  					unselState = FT.evalOption(
  						"unselectableStatus",
  						node,
  						node,
  						opts,
  						undefined
  					);
  					state = unselState == null ? !!node.selected : !!unselState;
  				}
  				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.