# Apply group
	#
	# https://docs.mongodb.com/manual/reference/operator/aggregation/group/
	#
	# Groups documents by some specified expression and outputs to the next stage
	# a document for each distinct grouping.
	#
	# The output documents contain an `_id` field which contains the distinct
	# group by key.
	#
	# The output documents can also contain computed fields that hold the values
	# of some accumulator expression grouped by the `$group`'s `_id` field.
	# `$group` does not order its output documents.
	#
	# ~~~json
	# { $group: { <group> } }
	# ~~~
	fun group(group: MongoGroup): MongoPipeline do return add_stage("group", group)