code: function lookup(root, partial, options) {
  
  	let settings = options.settings || {};
  	let partialSource = _resolve(_normalize(_join(settings.partialsFolder, partial+'.ejs')));
  
  	var key = [root, partial, '.ejs'].join('-');
  
  	if (options.cache && cache[key]) {
  		return cache[key];
  	}
  
  	if (_exists(partialSource)) {
  		return options.cache ? cache[key] = partial : partialSource;
  	}
  
  	return null;
  }
docs: Lookup partial path from base path of current template:
  
  - partial `_<name>`
  - any `<name>/index`
  - non-layout `../<name>/index`
  - any `<root>/<name>`
  - partial `<root>/_<name>`
  
  Options:
  
  - `cache` store the resolved path for the view, to avoid disk I/O
  
  @param {String} root, full base path of calling template
  @param {String} partial, name of the partial to lookup (can be a relative path)
  @param {Object} options, for `options.cache` behavior
  @return {String}
  @api private