hidden Curl

There are a few places where Curl code is documented as for internal use only*.  This will surely not be the case in open source Curl projects.

There are also some examples of the use of hidden as an access modifier such as this docs entry:

GenericIterator (class)

  hidden abstract GenericIterator
  Package: CURL.IO.STREAM

Another curious example is the parent of HVBox.

LengthBreadthBox (class)

  hidden abstract sealed LengthBreadthBox {inherits SequenceBox}
  Package: CURL.GUI.BASE

The documented 'hidden' is that for the hidden-package accessors in PackageNamespace and the field hiidden? in AliasedNameRef (which is itself internal-use only.)What makes this interesting is that SequenceBox is itself visible.
*example from Curl docs:
 seqnum-generator (field)
protected seqnum-generator:{proc-type {}:int}
Class: TableOptionItem
Package: CURL.GUI.BASE
... This item is unsupported and reserved for internal use.