This is one of those days I'd love to use multiple inheritance.
An AST_Class has lots of common with AST_Object, but unfortunately
`instanceof AST_Scope` is used very, very much, and a class has its name
inside its own special pocket scope. This compels me to make AST_Class
inherit Scope instead.
It looks like, although there is much in common with AST_Object,
`instanceof AST_Object` seldom are made, perhaps because it is less
often necessary to traverse an object than a scope.
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