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