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interesting idea. I was able to get it to work but there are some quirks you have to work around.

The var name MUST MATCH the map name in the inner predicate. And then if you want to have multiple uses in a single FS then you need to add a string variable otherwise it flags the inner predicate as a duplicate.

annotation { "Feature Type Name" : "test" }

export const test = defineFeature(function(context is Context, id is Id, definition is map)

precondition

{

      annotation { "Name" : "Widgets", "Item name" : "Widget" }
        definition.myWidgets is array;
        for (var widget in definition.myWidgets)
        {
            testPredicate(definition, widget, "0"); // this is the only declaration
        }
annotation { "Name" : "Widgets", "Item name" : "Widget" }
        definition.myWidgets2 is array;
        for (var widget in definition.myWidgets2)
        {
            testPredicate(definition, widget, "1"); // this is the only declaration
        }

export predicate testPredicate(definition is map, widget is map, i is string)

{

  annotation { "Name" : "My Length" }    
  isLength(widget["myLength" ~ i], LENGTH_BOUNDS);

}