Error: You are trying to perform a generic derivation of
  an expanded base class with at least one actual generic
  type parameter which is not `generic-creation-ready'.
What to do: `Generic-creation-readiness' states that each
  actual type parameter either meets his creation constraint
  and in case it is itself expanded is recursively
  `generic-creation-ready'.

  As an expanded type is automatically initialized one has
  to ensure, that all creation constraints are met.
  Consider GENERATOR [G create default_create end]
  This generator can create objects of type G. However,
  it is perfectly valid to use a type such as
  GENERATOR [COMPARABLE] as long as one does not try to create
  an object of this type directly. For expanded base classes
  this is not the case, as they are auto initialized.