# Define a custom operaton for `field`
	#
	# If no `field` is specified, append the operator the the root object:
	# ~~~json
	# {$<name>: <value>}
	# ~~~
	#
	# Else, append the operator to the field:
	# ~~~json
	# {field: {$<name>: <value>} }
	# ~~~
	fun op(name: String, field: nullable String, value: nullable Serializable): MongoMatch do
		if field != null then
			var q = new JsonObject
			q["${name}"] = value
			self[field] = q
		else
			self["${name}"] = value
		end
		return self
	end