docs: function _getKey(item) {
  
  	if (typeof item === null) {
  		throw new Error('adt-binary-search-tree utils.getKey(): item is null');
  	}
  
  	if (typeof item == 'object' && !item.hasOwnProperty('key')) {
  		throw new Error('adt-binary-search-tree utils.getKey(): item has no key');
  	}
  
  	if (typeof item == 'object' && item.key === null) {
  		throw new Error('adt-binary-search-tree utils.getKey(): item is null');
  	} else if (typeof item == 'object' && typeof item.key == 'object') {
  		throw new Error('adt-binary-search-tree utils.getKey(): item.key is an object');
  	} else if (typeof item == 'object') {
  		return item.key;
  	} else {
  		return item;
  	}
  
  }