tuple construction not working in activity generator

[abstratt]

Expected:

        return Expense extent.select((e : Expense) : Boolean {
            e.status == Status#Submitted
        }).groupBy((e : Expense) : Category { e.category }).groupCollect((group : Expense[*]) : {category : Category, count : Integer} {
            {
                category := group.one().category, 
                count := group.size()
            }
        });

Actual:

                return Expense extent.select((e) {
                	e.status == null
                }).groupBy((e) {
                	e.category
                }).groupCollect((group) {
                	begin
                	end
                });