# Push to a unique array
	#
	# https://docs.mongodb.com/manual/reference/operator/aggregation/addToSet/
	#
	# ~~~json
	# { $addToSet: <expression> }
	# ~~~
	#
	# Returns an array of all unique values that results from applying an
	# expression to each document in a group of documents that share the same
	# group by key.
	#
	# Order of the elements in the output array is unspecified.
	fun addToSet(field: String, expr: Serializable): MongoGroup do
		return acc("addToSet", field, expr)
	end