# Return the first value
	#
	# https://docs.mongodb.com/manual/reference/operator/aggregation/first/
	#
	# ~~~json
	# { $first: <expression> }
	# ~~~
	#
	# Returns the value that results from applying an expression to the first
	# document in a group of documents that share the same group by key.
	#
	# Only meaningful when documents are in a defined order.
	fun first(field: String, expression: Serializable): MongoGroup do
		return acc("first", field, expression)
	end