# Match documents where `field` exists or not
	#
	# https://docs.mongodb.com/manual/reference/operator/query/exists/#op._S_exists
	#
	# ~~~json
	# {field: {$exists: boolean} }
	# ~~~
	#
	# When `exists` is true, `$exists` matches the documents that contain the
	# field, including documents where the field value is null.
	# If <boolean> is false, the query returns only the documents that do not
	# contain the field.
	fun exists(field: String, exists: Bool): MongoMatch do
		op("exists", field, exists)
		return self
	end