Verifies that one frame is a direct subclass of another, or returns all
direct subclasses of a class.
- A class.
- Either a frame to be tested or a variable to be bound.
- If the :INCLUSIVE keyword is present and the subclass argument
is a variable, the superclass will be included as one of the
bindings of the variable.
(:DIRECT-SUBCLASS Physical_Objects Canines)
(:DIRECT-SUBCLASS Mammals ?what)
(:DIRECT-SUBCLASS Mammals ?what :INCLUSIVE)
The superclass argument must be a class.
If the clause is ground, succeeds if the relationship is true.
If the clause is not ground, succeeds if it retrieves at least one direct subclass.
Fails if the superclass is not a class, if the direct-subclass relationship is not true,
or if the parent has no direct subclasses.
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