GRASP is a  fairly large project (over 22k lines of code in 77 files), but
I ran into a problem.

Until recently, I had an interface called Tile in the (editor interfaces
elements) module, defined in the following way:

(define-interface Tile (Element)
  (extent)::Extent)

where define-interface is a macro that would expand to

(define-simple-class Tile (Element) interface: #t
  ((extent)::Extent #!abstract))

The (editor interfaces elements) module imports the module (editor
interfaces painting), and they are used in various places in the project.
Among others, those interfaces are implemented by the Atom class, which is
defined in the (editor types primitive) module (which imports both
aforementioned modules)

I tried refactoring this, by defining  a new interface:

(define-interface Extensive ()
  (extent)::Extent)

in the (editor interfaces painting) module, and changing the definition of
Tile to

(define-interface Tile (Extensive Element))

But when I do this, the system claims that Atom is not Extensive (even
though it implements a sub-interface of the Tile interface). How could that
be?

Also, during compilation I get warnings like this from one of the modules:

*type editor.interfaces.Painter is incompatible with required type
editor.interfaces.Painter.*

How are they supposed to be understood?