code: function plugin(cb) {
  	return function(css) {
  		if (lodash.isFunction(cb)) {
  			var token = cb(css.source.input.file);
  
  			if (!lodash.isNull(token)) {
  				css.eachRule(function(rule) {
  					if (!areSelectorsValid(rule.selectors, token)) {
  						throw rule.error('Wrong selector');
  					}
  				});
  			}
  		}
  	}
  }
docs: PostCSS plugin definition.
  
  @param  {Function} cb
  @return {Function}