* (defclass a () ())
#<STANDARD-CLASS COMMON-LISP-USER::A>
* (defclass b (a a) ())

debugger invoked on a SIMPLE-ERROR in thread
#<THREAD "main thread" RUNNING {10008A8083}>:
  While computing the class precedence list of the class named COMMON-LISP-USER::B.
It is not possible to compute the class precedence list because
there is a circularity in the local precedence relations.
This arises because:
  The class named COMMON-LISP-USER::A follows the class named COMMON-LISP-USER::A in the supers of the class named COMMON-LISP-USER::B..

Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [ABORT] Exit debugger, returning to top level.

(SB-PCL::CPL-ERROR #<STANDARD-CLASS COMMON-LISP-USER::B {100319A013}> "It is not possible to compute the class precedence list because~@
       there ~A in the local precedence relations.~@
       ~A because:~{~%  ~A~}." "is a circularity" "This arises" ("The class named COMMON-LISP-USER::A follows the class named COMMON-LISP-USER::A in the supers of the class named COMMON-LISP-USER::B."))