Sum of one product in a list of products

How to get from below collection with multiple product_ID’s the sum of one product_ID that can have multiple entrances in the list)

		'lijsten': collection ['post'] {
			'post': text @default: auto-increment || "01"
			'klant': text
			'product_ID': text -> ^ ^ .'Producten'.'Producten'[]
			'productNr': number 'ID' = >'product_ID'.'productNr' @hidden
			'product': text
			'aantal': number 'aantal'

found already solution

Does the solution use the ‘union’ construction for derived collections? That is what I would use - it is similar to a SQL Group By (but more powerful and much more safely typed).