# Add an accumulator
	#
	# Each accumulator is registered as:
	# ~~~json
	# <field>: { <accumulator> : <expression> }
	# ~~~
	private fun acc(name: String, field: String, expression: nullable Serializable): MongoGroup do
		var q = new JsonObject
		q["${name}"] = expression
		self[field] = q
		return self
	end