# Calculates and returns the sum of numeric values
	#
	# https://docs.mongodb.com/manual/reference/operator/aggregation/sum/#grp._S_sum
	#
	# ~~~json
	# { $sum: <expression> }
	# ~~~
	#
	# `$sum` ignores non-numeric values.
	fun sum(field: String, expression: Serializable): MongoGroup do
		return acc("sum", field, expression)
	end