         /// This form accepts a Document for more flexiblity.
         ///
         /// https://docs.mongodb.com/manual/reference/operator/aggregation/group/
-        @discardableResult
         public static func group(groupDocument: Document) -> Stage {
             return Stage([
                 "$group": groupDocument

        }
        
        /// Performs a left outer join to an unsharded collection in the same database
        @discardableResult
        public static func lookup(from collection: Collection, localField: String, foreignField: String, as: String) -> Stage {
            return Stage([
                "$lookup": [

        }
        
        /// Takes a sample with the size of `size`. These randomly selected Documents will be passed to the next stage.
        public static func sample(sizeOf size: Int) -> Stage {
            return Stage([
                "$sample": ["size": size]
