Using IN to decouple object name coordination

Suppose you have a situation in which you do not want to coordinate common objects. You might, for example, want to count different objects contained in the same object, as in:

Summary = COUNTabs ( Header:Order:Message ) + COUNTabs ( Line Item:Order:Message )

If there are multiple Orders in your data, the Summary would not be evaluated correctly. This rule would only count the Headers of the first Order and add it to the count of all Line Items of the first Order. To count all the Headers and all the Line Items in all the Orders, use IN:

Summary = COUNTabs ( Header IN Message) + COUNTabs ( Line Item IN Message )