tag field narrowing does not work with branded types

[sakari]

branded types cannot be used for narrowing. Say that we have a

TagA:
 type: string
  enum:
    - a
A: 
 oneOf:
  - TaggedA
  -  ...
TaggedA:
  type: object
  properties:
    tag: 
      $ref: TagA

then typescript does not narrow by the tag field as the tag value gets branded as a top level scalar definition.

function foo(a: A): null | Value {
  if (tagged.tag === 'a') {
    return a.value;
  }
  return null;
}

instead need to use instanceof TaggedA or work with ShapeOfTaggedA. Or if possible define TaggedA as

TaggedAd:
...
properties:
  tag: 
    type: string
    enum: a
...