using concurrent

class Tree{}
class AdvancedTree:Tree{}

mixin HasTree{
  virtual Tree tree(){
    Actor.locals["tree"]
  }
}

mixin HasAdvancedTree : HasTree{
  override AdvancedTree tree(){
    super.tree
  }
}

Compiler output:

Test.fan(15,11): Unknown slot 'sys::Obj.tree'

I guess because you don't inherit from Mixins, so super.tree() doesn't exist in Obj.

super.foo means in my superclass, you can use named mixin like Tree.super.tree