mongo aggregation fluent API

gistfile1.js

//Usage
//   example: unwind "tags" field, group by tag + count, save output to collection "FOO"
//   db.things.agg().unwind("tags").group("tags", {total:{$sum:1}}).out("FOO").exec()

var AggHelper = function(collection){
    this.collection = collection
    this.db = collection.getDB()
    this.pipeline = []
}

AggHelper.prototype.match = function(criteria){
    if(!criteria){
        throw "match needs a query object, like {'name' : 'bob'}"
    }
    this.pipeline.push({"$match" : criteria})
    return this;
}

AggHelper.prototype.group = function(id, exprs){
    if(!id){
        throw "group needs an 'id' (grouping key) and a set of group operators, like {totalAge:{'$sum':'$age'}}"
    }
    groupexprs = exprs
    groupexprs._id = "$" + id
    this.pipeline.push({"$group" : groupexprs})
    return this;
}

AggHelper.prototype.sort = function(info){
    this.pipeline.push({"$sort" : info})
    return this;
}

AggHelper.prototype.skip = function(n){
    if(!n){
        throw "skip needs an integer indicating the number of documents to skip."
    }
    this.pipeline.push({"$skip" : n})
    return this;
}

AggHelper.prototype.limit = function(n){
    if(!n){
        throw "limit needs an integer indicating the max number of documents to limit."
    }
    this.pipeline.push({"$limit" : n})
    return this;
}

AggHelper.prototype.unwind = function(fieldname){
    if(!fieldname){
        throw "unwind needs a string indicating the key of an array field to unwind"
    }
    this.pipeline.push({"$unwind" : "$" + fieldname})
    return this;
}

AggHelper.prototype.toString = function(){
    return tojson(this.pipeline)
}



AggHelper.prototype.project = function(fieldsinfo){
    if(!fieldsinfo){
        throw "project needs info about what to project"
    }
    this.pipeline.push({"$project" : fieldsinfo})
    return this;
}

AggHelper.prototype.project = function(fieldsinfo){
    if(!fieldsinfo){
        throw "project needs info about what to project"
    }
    this.pipeline.push({"$project" : fieldsinfo})
    return this;
}

AggHelper.prototype.out = function(out_collection_name){
    if(!out_collection_name){
        throw "out() needs a string for the name of the collection to save output to."
    }
    this.out_collection_name = out_collection_name
    return this;
}

AggHelper.prototype.exec = function(){
    var result = this.collection.aggregate(this.pipeline)
    if(this.out_collection_name){
        var output_collection = this.db.getCollection(this.out_collection_name)
        for(var i=0;i<result.result.length;i++){
            output_collection.insert(result.result[i])
        }
    }
    return result;
}

DBCollection.prototype.agg = function(){
    return new AggHelper(this)
}

but how do I mongo?