Check if there is a bug related to multiple superclasses

[hmuniz]

https://github.com/own-pt/cl-krr/blob/2d0133217745bbd90e46468abb9da24ebdf407e5/suo-kif.lisp#L120

[arademaker]

This issue is:

• [ ] confirm if the current version is handling multiple inheritance
• [ ] fix the code if not, a class can have more than one superclass