It should be a set according to http://www.alu.org/mop/dictionary.html *Generic Function* *class-direct-subclasses* *class* Returns a set of the direct subclasses of *class*. The elements of this set are class metaobjects that all mention this class among their direct superclasses. The empty set is returned if *class* has no direct subclasses. This value is maintained by the generic functions *add-direct-subclass*<http://www.alu.org/mop/dictionary.html#add-direct-subclass> and *remove-direct-subclass*<http://www.alu.org/mop/dictionary.html#remove-direct-subclass> . (map nil 'print (system:class-direct-subclasses (find-class 'ido-pathway-block))) #<STANDARD-CLASS PARSED-SHEET-INFO-BLOCK {895391}> #<STANDARD-CLASS PARSED-EVIDENCE-BLOCK {C422A5}> #<STANDARD-CLASS PARSED-COMPLEX-BLOCK {90AD53}> #<STANDARD-CLASS PARSED-PROCESS-BLOCK {14D509}> #<STANDARD-CLASS PARSED-HANDLE-BLOCK {5AFFB3}> #<STANDARD-CLASS PARSED-SHEET-INFO-BLOCK {895391}> #<STANDARD-CLASS PARSED-EVIDENCE-BLOCK {C422A5}> #<STANDARD-CLASS PARSED-COMPLEX-BLOCK {90AD53}> #<STANDARD-CLASS PARSED-PROCESS-BLOCK {14D509}> #<STANDARD-CLASS PARSED-HANDLE-BLOCK {5AFFB3}> -Alan