# Add a stage to the pipeline
	#
	# https://docs.mongodb.com/manual/reference/operator/aggregation/#stage-operators
	#
	# Each stage is registered as:
	# ~~~json
	# { $<stage>: <json> }
	# ~~~
	fun add_stage(stage: String, json: Serializable): MongoPipeline do
		var obj = new JsonObject
		obj["${stage}"] = json
		add obj
		return self
	end