PK_ERROR_code_t     PK_CLASS_is_subclass
(
--- received arguments ---
PK_CLASS_t          may_be_subclass,--- a potential subclass
PK_CLASS_t          class,          --- a class

--- returned arguments ---
PK_LOGICAL_t *const is_subclass     --- whether it was a subclass
)


This function returns whether may_be_subclass is a subclass of class.


The return argument is PK_LOGICAL_true if succession of calls to
PK_CLASS_ask_superclass lead from may_be_subclass to class.

PK_CLASS_class is not a subclass of any class.  All other classes are
subclasses of PK_CLASS_class.