(cl:comment
 'Alexander died of polonium poisoning.'
 (poisoned-by-polonium Alexander))

(cl:comment
 'Yuri is a suspect.'
 (suspect Yuri))

(cl:comment
 'Dan is a suspect.'
 (suspect Dan))
  
(cl:comment
 'Josef is a suspect.'
 (suspect Josef))

(cl:comment
 'Yuri\'s living quarters were found to have traces of polonium.'
 (polonium (living-quarters Yuri)))
  
(cl:comment
 'Dan\'s living quarters were found to be clear of polonium.'
 (not (polonium (living-quarters Dan))))
  
(cl:comment
 'Josef\'s living quarters were found to be clear of polonium.'
 (not (polonium (living-quarters Josef))))

(cl:comment
 'Tacit Information'
 (and (cl:comment
       'The people are distinct individuals.'
       (and (not (= Alexander Yuri))
            (not (= Alexander Josef))
            (not (= Alexander Dan))
            (not (= Josef Dan))
            (not (= Josef Yuri))
            (not (= Dan Yuri))))
      (cl:comment
       'Alexander did not kill himself.'
       (not (killed Alexander Alexander)))
      (cl:comment
       'Alexander\'s living quarters did not
        contain traces of polonium.'
       (not (polonium (living-quarters Alexander))))
      (cl:comment
       'Alexander is the only person who has been killed.'
       (forall (x) (if (exists (y) (killed y x))
                     (= Alexander x))))
      (cl:comment
       'A person is killed by only one other person.'
       (forall (x y z) (if (and (killed x z)
                                (killed y z))
                         (= x y))))
      (cl:comment
       'Being poisoned by polonium entails having been killed.'
       (forall (x) (if (poisoned-by-polonium x)
                     (exists (y) (killed y x)))))
      (cl:comment
       'No one shares living quarters.'
       (forall (w y z) (if (and (= (living-quarters w) z)
                                (= (living-quarters y) z))
                         (= w y))))))