code: function replaceDecls(originalRules, criticalRule){
  		// find all the rules in the original CSS that have the same selectors and
  		// then create an array of all the associated declarations. Note that this
  		// works with mutiple duplicate selectors on the original CSS
  		var originalDecls = _.flatten(
  			originalRules
  				.filter(function(rule){
  					return _.isEqual(rule.selectors, criticalRule.selectors);
  				})
  				.map(function(rule){
  					return rule.declarations;
  				})
  		);
  
  		// take all the declarations that were found from the original CSS and use
  		// them here, make sure that we de-dup any declarations from the original CSS
  		criticalRule.declarations =
  			_.uniqBy(criticalRule.declarations.concat(originalDecls),	function(decl){
  				return decl.property + ":" + decl.value;
  			});
  
  		return criticalRule;
  	}
docs: create a function that can be passed to `map` for a collection of critical css rules. The function will match original rules against the selector of the critical css declarations, concatenate them together, and then keep only the unique ones