A common use case of the virtual type SELF is to type an attribute and
store another instance of the same type as self. It can also be used as as
return type to a method producing a copy of self or returning an instance
expected to be the exact same type as self.
This virtual type must be used with caution as it can hinder specialization.
In fact, it imposes strict restrictions on all sub-classes and their usage.
For example, using SELF as a return type of a method foo
forces all subclasses to ensure that foo returns the correct and updated
type.
A dangerous usage take the form of a method typed by SELF which creates
and returns a new instance.
If not correctly specialized, this method would break when invoked on a
sub-class.
A general rule for safe usage of SELF is to ensure that inputs typed
SELF are stored in attributes typed SELF and returned by methods typed
SELF, pretty much the same things as you would do with parameter types.