The following, which I thought would not work, does work:
 
c1 = .MyClass1~new;  c1~doIt
c2 = .MyClass2~new;  c2~doIt
 
::class MyClass1 inherit  MixinA
::class MyClass2 subclass MixinA
 
::class MixinA mixinclass object;
  ::method doIt;  say "MixinA caught it."
 
/* Command Prompt Output:
MixinA caught it.
MixinA caught it.
*/

Is this working to spec? There's no mention (that I can find) in the ooRexx Reference about sublassing a mixin - as opposed to inheriting it.
I hope it is working to spec, 'cos I really do like it! 

Thanks,

--
Oliver Sims