Whenever you need to give a type of something in the dialog, the type selection component is used. The type selection lets you type any class name or you can pick one from the list.
When you select a class that is in the system and has formal generic parameters, the type selection component lets you specify an actual generic parameter in another type selection.
Since the actual generic type selection is also a type selection, you can select another class with generic parameters and nest them as deep as you need.
You can also select the type
The code generated for the selected type is quite straightforward, an example:
new_feature: FUNCTION [TUPLE [INTEGER], BOOLEAN]