The RootDoc.classes() and PackageDoc.allClasses() mean
by "class" -- an interface, error, exception, or ordinary
class.

Doc.isClass() returns true if the item is a
"class", but not an interface. (It's not clear whether it
includes errors and exceptions, or not.)

Throughout the API, the word "class" needs to be clearly described
as to whether it means "class and interface" or "class, not interface".
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If we want clarity I suggest renaming:
  RootDoc.classes() to RootDoc.classesAndInterfaces()
  PackageDoc.allClasses() to allClassesAndInterfaces()

  xxxxx@xxxxx   2002-09-29