Chapter 4, Metalinguistic Abstraction

Exercise 4.67


(I implemented this solution after doing ex-4.76)

This took me quite some time but the solution turned out quite simple.

This solution can be optimized further. As this took me already quite some time, I will just leave it at this.

Let me first write the way I arrived at the solution:

  • Check how the bindings are happening by considering the “marriage” rule example of infinite loop given in the book.
  • Loop is possible only by rule application.
  • Think of rule application as procedure invocation.
  • The loop occurs when the procedure/rule is called with same values for the formal-arguments/variables!

  • (This point is specific to my solution)Store every time the procedure/rule is invoked with the binding of the variables/args of the rule/procedure. Thus store the rule/procedure conclusion/declaration with its arguments.
  • (Specific to my solution)Maintain history similar to frames. Pass history by adding items in the beginning of history thus no need for assignments or history removal when rule gets applied.

Solution:

Just maintain the history of instantiated rule conclusions and replace the variables which are left after instantiation by ‘??.

Code:

(Complete code is at the bottom of this page)

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(define (make-empty-history) '())

(define (make-history-item pattern frame)
  (instantiate pattern
			   frame
			   (lambda (v f) '??)))

(define (in-history? item history)
  (if (null? history)
	  #f
	  (if (equal? item (car history))
		  #t
		  (in-history? item (cdr history)))))

(define (add-item-in-history item history)
  (cons item history))

(define (apply-a-rule rule query-pattern query-frame history)
  (let ((clean-rule (rename-variables-in rule)))
    (let ((unify-result
           (unify-match query-pattern
                        (conclusion clean-rule)
                        query-frame)))
      (if (eq? unify-result 'failed)
          the-empty-stream
		  (let ((new-history-item
				 (make-history-item (conclusion clean-rule)
									unify-result)))
			(if (in-history? new-history-item history)
				(begin
				  (print-items "Loop Detected!" " the history item " new-history-item " is repeated " history)
				  the-empty-stream)
				(qeval (rule-body clean-rule)
					   (singleton-stream unify-result)
					   (add-item-in-history new-history-item history))))))))

Test/Example:

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;;; Query input:

(assert! (rule (append-to-form () ?y ?y)))
(assert! (rule (append-to-form (?u . ?v) ?y (?u . ?z))
			   (append-to-form ?v ?y ?z)))

;;Created to rules for reverse, for testing(the difference is the order of clauses containing append-to-form)
(assert! (rule (reversal () ())))
(assert! (rule (reversal (?x . ?y) ?r)
			   (and (reversal ?y ?ry)
					(append-to-form ?ry (?x) ?r))))

(assert! (rule (reverse () ())))
(assert! (rule (reverse (?x . ?y) ?r)
			   (and (append-to-form ?ry (?x) ?r)
					(reverse ?y ?ry))))

Assertion added to data base.
;...
Assertion added to data base.

;;; Query input:
(reverse (1 2 3) ?x)

;;; Query results:
(Loop Detected!  the history item  (append-to-form (?? . ??) (1) (?? . ??))  is repeated ((append-to-form (?? . ??) (1) (?? . ??)) (reverse (1 2 3) ??)))

;;; Query input:
(reversal (1 2 3) ?x)

;;; Query results:
(reversal (1 2 3) (3 2 1))

;;; Query input:
(reversal ?x (1 2 3))

;;; Query results:
(Loop Detected!  the history item  (reversal (?? . ??) ??)  is repeated ((reversal (?? . ??) ??) (reversal (?? . ??) (1 2 3))))


;;;The example given in book
;;; Query input:
(assert! (married Minnie Mickey))

Assertion added to data base.

;;; Query input:
(married Minnie ?who)

;;; Query results:
(married minnie mickey)

;;; Query input:
(married Mickey ?who)

;;; Query results:
;;empty

;;; Query input:
(assert! (rule (married ?x ?y)
               (married ?y ?x)))

Assertion added to data base.

;;; Query input:
(married Mickey ?who)

;;; Query results:
(married mickey minnie)
(Loop Detected!  the history item  (married mickey ??)  is repeated ((married ?? mickey) (married mickey ??)))

;;; Query input:

Possible Optimization

Instead of storing instantiated pattern, story the conclusion with variables. Store a list for each conclusion whose each element contains the list of values/arguments of the variables for each invocation.

I started this but stopped because of problems with assignments. Check a partial solution in the complete code(commented - present under optimized heading in the code).

The problem is to revert the assignment, thus inside the apply-a-rule, when the rule evaluation completes, i should remove the history also that was added because of rule evaluation.


Complete code:

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;tom;;;QUERY SYSTEM FROM SECTION 4.4.4 OF
;;;; STRUCTURE AND INTERPRETATION OF COMPUTER PROGRAMS

;;;;Matches code in ch4.scm
;;;;Includes:
;;;;  -- supporting code from 4.1, chapter 3, and instructor's manual
;;;;  -- data base from Section 4.4.1 -- see microshaft-data-base below

;;;;This file can be loaded into Scheme as a whole.
;;;;In order to run the query system, the Scheme must support streams.

;;;;NB. PUT's are commented out and no top-level table is set up.
;;;;Instead use initialize-data-base (from manual), supplied in this file.


;;;SECTION 4.4.4.1
;;;The Driver Loop and Instantiation

(define input-prompt ";;; Query input:")
(define output-prompt ";;; Query results:")

(define (query-driver-loop)
  (prompt-for-input input-prompt)
  (let ((q (query-syntax-process (read))))
    (cond ((assertion-to-be-added? q)
           (add-rule-or-assertion! (add-assertion-body q))
           (newline)
           (display "Assertion added to data base.")
           (query-driver-loop))
          (else
           (newline)
           (display output-prompt)
           ;; [extra newline at end] (announce-output output-prompt)
           (display-stream
            (stream-map
             (lambda (frame)
               (instantiate q
                            frame
                            (lambda (v f)
                              (contract-question-mark v))))
             (qeval q (singleton-stream '()) (make-empty-history))))
           (query-driver-loop)))))

(define (instantiate exp frame unbound-var-handler)
  (define (copy exp)
    (cond ((var? exp)
           (let ((binding (binding-in-frame exp frame)))
             (if binding
                 (copy (binding-value binding))
                 (unbound-var-handler exp frame))))
          ((pair? exp)
           (cons (copy (car exp)) (copy (cdr exp))))
          (else exp)))
  (copy exp))


;;;SECTION 4.4.4.2
;;;The Evaluator

(define (qeval query frame-stream history)
  (let ((qproc (get (type query) 'qeval)))
    (if qproc
        (qproc (contents query) frame-stream history)
        (simple-query query frame-stream history))))

;;;Simple queries

(define (simple-query query-pattern frame-stream history)
  (stream-flatmap
   (lambda (frame)
     (stream-append-delayed
      (find-assertions query-pattern frame)
      (delay (apply-rules query-pattern frame history))))
   frame-stream))

;;;Compound queries

(define (conjoin conjuncts frame-stream history)
  (if (empty-conjunction? conjuncts)
      frame-stream
      (conjoin (rest-conjuncts conjuncts)
               (qeval (first-conjunct conjuncts)
                      frame-stream
					  history)
			   history)))

;; ex-4.76
;; (define (optimized-conjoin conjuncts frame-stream)
;;   (define (iter conjuncts merged-stream)
;; 	(if (empty-conjunction? conjuncts)
;; 		merged-stream
;; 		(iter (rest-conjuncts conjuncts)
;; 			  (merge-frame-stream
;; 			   (qeval (first-conjunct conjuncts)
;; 					  frame-stream)
;; 			   merged-stream))))
  
;;   (stream-filter
;;    (lambda (frame) (not (eq? frame 'failed)))
;;    (iter conjuncts frame-stream)))

;; (define (merge-frame-stream stream1 stream2)
;;   (stream-flatmap (lambda (frame1)
;; 					(stream-map (lambda (frame2)
;; 								  (merge-frames frame1 frame2))
;; 								stream2))
;; 				  stream1))

;; (define (merge-frames frame1 frame2)
;;   (cond ((or (eq? 'failed frame1)
;; 			 (eq? 'failed frame2))
;; 		 'failed)
;; 		((null? frame1) frame2) 
;; 		(else (merge-frames (cdr frame1)
;; 								   (extend-if-possible (caar frame1)
;; 													   (cdar frame1)
;; 													   frame2)))))
;;(put 'and 'qeval conjoin)


(define (disjoin disjuncts frame-stream history)
  (if (empty-disjunction? disjuncts)
      the-empty-stream
      (interleave-delayed
       (qeval (first-disjunct disjuncts) frame-stream history)
       (delay (disjoin (rest-disjuncts disjuncts)
                       frame-stream
					   history)))))

;;(put 'or 'qeval disjoin)

;;;Filters

;; ex-4.75 
(define (unique-query exps) (car exps))

(define (uniquely-asserted operands frame-stream history)
  (simple-stream-flatmap
   (lambda (frame)
	 (let ((matched-frame-stream (qeval (unique-query operands)
										(singleton-stream frame)
										history)))
	   (if (or (stream-null? matched-frame-stream)
			   (not (stream-null? (stream-cdr matched-frame-stream))))
		   the-empty-stream
		   matched-frame-stream)))
   frame-stream))

;; (put 'unique 'qeval uniquely-asserted)

(define (negate operands frame-stream history)
  (simple-stream-flatmap
   (lambda (frame)
     (if (stream-null? (qeval (negated-query operands)
                              (singleton-stream frame)
							  history))
         (singleton-stream frame)
         the-empty-stream))
   frame-stream))

;;(put 'not 'qeval negate)

(define (lisp-value call frame-stream history)
  (simple-stream-flatmap
   (lambda (frame)
     (if (execute
          (instantiate
           call
           frame
           (lambda (v f)
             (error "Unknown pat var -- LISP-VALUE" v))))
         (singleton-stream frame)
         the-empty-stream))
   frame-stream))

;;(put 'lisp-value 'qeval lisp-value)

(define (execute exp)
  (apply (eval (predicate exp) user-initial-environment)
         (args exp)))

(define (always-true ignore frame-stream history) frame-stream)

;;(put 'always-true 'qeval always-true)

;;;SECTION 4.4.4.3
;;;Finding Assertions by Pattern Matching

(define (find-assertions pattern frame)
  (simple-stream-flatmap (lambda (datum)
                    (check-an-assertion datum pattern frame))
                  (fetch-assertions pattern frame)))

(define (check-an-assertion assertion query-pat query-frame)
  (let ((match-result
         (pattern-match query-pat assertion query-frame)))
    (if (eq? match-result 'failed)
        the-empty-stream
        (singleton-stream match-result))))

(define (pattern-match pat dat frame)
  (cond ((eq? frame 'failed) 'failed)
        ((equal? pat dat) frame)
        ((var? pat) (extend-if-consistent pat dat frame))
        ((and (pair? pat) (pair? dat))
         (pattern-match (cdr pat)
                        (cdr dat)
                        (pattern-match (car pat)
                                       (car dat)
                                       frame)))
        (else 'failed)))

(define (extend-if-consistent var dat frame)
  (let ((binding (binding-in-frame var frame)))
    (if binding
        (pattern-match (binding-value binding) dat frame)
        (extend var dat frame))))

;;;history maintenance for loop detector
;;;in ex-4.67

(define (make-empty-history) '())

(define (make-history-item pattern frame)
  (instantiate pattern
			   frame
			   (lambda (v f) '??)))

(define (in-history? item history)
  (if (null? history)
	  #f
	  (if (equal? item (car history))
		  #t
		  (in-history? item (cdr history)))))

(define (add-item-in-history item history)
  (cons item history))

(define (print-items . items)
  (newline)
  (display items))

;; For optimization (try later)
;; (define (add-in-history! pattern frame history)
;;   (let ((history-item (make-history-item pattern frame)))
;; 	(let ((pattern (car history-item))
;; 		  (val (cdr history-item)))
;; 	  (let ((cons-containing-pattern (assoc pattern history)))
;; 		(if cons-containing-pattern
;; 			(let ((instantiated-values (cdr cons-containing-pattern)))
;; 			  (if (memq val instantiated-values)
;; 				  #f
;; 				  (begin
;; 					(set-cdr! cons-containing-pattern
;; 							  (cons val instantiated-values))
;; 					history)))
;; 			(cons
;; 			 (cons pattern
;; 				   (cons val '()))
;; 			 history))))))

;;end of history maintenance

;;;SECTION 4.4.4.4
;;;Rules and Unification

(define (apply-rules pattern frame history)
  (stream-flatmap (lambda (rule)
                    (apply-a-rule rule pattern frame history))
                  (fetch-rules pattern frame)))

(define (apply-a-rule rule query-pattern query-frame history)
  (let ((clean-rule (rename-variables-in rule)))
    (let ((unify-result
           (unify-match query-pattern
                        (conclusion clean-rule)
                        query-frame)))
      (if (eq? unify-result 'failed)
          the-empty-stream
		  (let ((new-history-item
				 (make-history-item (conclusion clean-rule)
									unify-result)))
			(if (in-history? new-history-item history)
				(begin
				  (print-items "Loop Detected!" " the history item " new-history-item " is repeated " history)
				  the-empty-stream)
				(qeval (rule-body clean-rule)
					   (singleton-stream unify-result)
					   (add-item-in-history new-history-item history))))))))

(define (rename-variables-in rule)
  (let ((rule-application-id (new-rule-application-id)))
    (define (tree-walk exp)
      (cond ((var? exp)
             (make-new-variable exp rule-application-id))
            ((pair? exp)
             (cons (tree-walk (car exp))
                   (tree-walk (cdr exp))))
            (else exp)))
    (tree-walk rule)))

(define (unify-match p1 p2 frame)
  (cond ((eq? frame 'failed) 'failed)
        ((equal? p1 p2) frame)
        ((var? p1) (extend-if-possible p1 p2 frame))
        ((var? p2) (extend-if-possible p2 p1 frame)) ; {\em ; ***}
        ((and (pair? p1) (pair? p2))
         (unify-match (cdr p1)
                      (cdr p2)
                      (unify-match (car p1)
                                   (car p2)
                                   frame)))
        (else 'failed)))

(define (extend-if-possible var val frame)
  (let ((binding (binding-in-frame var frame)))
    (cond (binding
           (unify-match
            (binding-value binding) val frame))
          ((var? val)                     ; {\em ; ***}
           (let ((binding (binding-in-frame val frame)))
             (if binding
                 (unify-match
                  var (binding-value binding) frame)
                 (extend var val frame))))
          ((depends-on? val var frame)    ; {\em ; ***}
           'failed)
          (else (extend var val frame)))))

(define (depends-on? exp var frame)
  (define (tree-walk e)
    (cond ((var? e)
           (if (equal? var e)
               true
               (let ((b (binding-in-frame e frame)))
                 (if b
                     (tree-walk (binding-value b))
                     false))))
          ((pair? e)
           (or (tree-walk (car e))
               (tree-walk (cdr e))))
          (else false)))
  (tree-walk exp))

;;;SECTION 4.4.4.5
;;;Maintaining the Data Base

(define THE-ASSERTIONS the-empty-stream)

(define (fetch-assertions pattern frame)
  (if (use-index? pattern)
      (get-indexed-assertions pattern)
      (get-all-assertions)))

(define (get-all-assertions) THE-ASSERTIONS)

(define (get-indexed-assertions pattern)
  (get-stream (index-key-of pattern) 'assertion-stream))

(define (get-stream key1 key2)
  (let ((s (get key1 key2)))
    (if s s the-empty-stream)))

(define THE-RULES the-empty-stream)

(define (fetch-rules pattern frame)
  (if (use-index? pattern)
      (get-indexed-rules pattern)
      (get-all-rules)))

(define (get-all-rules) THE-RULES)

(define (get-indexed-rules pattern)
  (stream-append
   (get-stream (index-key-of pattern) 'rule-stream)
   (get-stream '? 'rule-stream)))

(define (add-rule-or-assertion! assertion)
  (if (rule? assertion)
      (add-rule! assertion)
      (add-assertion! assertion)))

(define (add-assertion! assertion)
  (store-assertion-in-index assertion)
  (let ((old-assertions THE-ASSERTIONS))
    (set! THE-ASSERTIONS
          (cons-stream assertion old-assertions))
    'ok))

(define (add-rule! rule)
  (store-rule-in-index rule)
  (let ((old-rules THE-RULES))
    (set! THE-RULES (cons-stream rule old-rules))
    'ok))

(define (store-assertion-in-index assertion)
  (if (indexable? assertion)
      (let ((key (index-key-of assertion)))
        (let ((current-assertion-stream
               (get-stream key 'assertion-stream)))
          (put key
               'assertion-stream
               (cons-stream assertion
                            current-assertion-stream))))))

(define (store-rule-in-index rule)
  (let ((pattern (conclusion rule)))
    (if (indexable? pattern)
        (let ((key (index-key-of pattern)))
          (let ((current-rule-stream
                 (get-stream key 'rule-stream)))
            (put key
                 'rule-stream
                 (cons-stream rule
                              current-rule-stream)))))))

(define (indexable? pat)
  (or (constant-symbol? (car pat))
      (var? (car pat))))

(define (index-key-of pat)
  (let ((key (car pat)))
    (if (var? key) '? key)))

(define (use-index? pat)
  (constant-symbol? (car pat)))

;;;SECTION 4.4.4.6
;;;Stream operations

(define (stream-append-delayed s1 delayed-s2)
  (if (stream-null? s1)
      (force delayed-s2)
      (cons-stream
       (stream-car s1)
       (stream-append-delayed (stream-cdr s1) delayed-s2))))

(define (interleave-delayed s1 delayed-s2)
  (if (stream-null? s1)
      (force delayed-s2)
      (cons-stream
       (stream-car s1)
       (interleave-delayed (force delayed-s2)
                           (delay (stream-cdr s1))))))

(define (stream-flatmap proc s)
  (flatten-stream (stream-map proc s)))

(define (flatten-stream stream)
  (if (stream-null? stream)
      the-empty-stream
      (interleave-delayed
       (stream-car stream)
       (delay (flatten-stream (stream-cdr stream))))))

;; ex-4.74
(define simple-stream-flatmap stream-flatmap)

;; (define (simple-stream-flatmap proc s)
;;   (simple-flatten (stream-map proc s)))

;; (define (simple-flatten stream)
;;   (stream-map stream-car
;;               (stream-filter (lambda (s)
;;                                (not (stream-null? s)))
;;                              stream)))

(define (singleton-stream x)
  (cons-stream x the-empty-stream))


;;;SECTION 4.4.4.7
;;;Query syntax procedures

(define (type exp)
  (if (pair? exp)
      (car exp)
      (error "Unknown expression TYPE" exp)))

(define (contents exp)
  (if (pair? exp)
      (cdr exp)
      (error "Unknown expression CONTENTS" exp)))

(define (assertion-to-be-added? exp)
  (eq? (type exp) 'assert!))

(define (add-assertion-body exp)
  (car (contents exp)))

(define (empty-conjunction? exps) (null? exps))
(define (first-conjunct exps) (car exps))
(define (rest-conjuncts exps) (cdr exps))

(define (empty-disjunction? exps) (null? exps))
(define (first-disjunct exps) (car exps))
(define (rest-disjuncts exps) (cdr exps))

(define (negated-query exps) (car exps))

(define (predicate exps) (car exps))
(define (args exps) (cdr exps))


(define (rule? statement)
  (tagged-list? statement 'rule))

(define (conclusion rule) (cadr rule))

(define (rule-body rule)
  (if (null? (cddr rule))
      '(always-true)
      (caddr rule)))

(define (query-syntax-process exp)
  (map-over-symbols expand-question-mark exp))

(define (map-over-symbols proc exp)
  (cond ((pair? exp)
         (cons (map-over-symbols proc (car exp))
               (map-over-symbols proc (cdr exp))))
        ((symbol? exp) (proc exp))
        (else exp)))

(define (expand-question-mark symbol)
  (let ((chars (symbol->string symbol)))
    (if (string=? (substring chars 0 1) "?")
        (list '?
              (string->symbol
               (substring chars 1 (string-length chars))))
        symbol)))

(define (var? exp)
  (tagged-list? exp '?))

(define (constant-symbol? exp) (symbol? exp))

(define rule-counter 0)

(define (new-rule-application-id)
  (set! rule-counter (+ 1 rule-counter))
  rule-counter)

(define (make-new-variable var rule-application-id)
  (cons '? (cons rule-application-id (cdr var))))

(define (contract-question-mark variable)
  (string->symbol
   (string-append "?" 
     (if (number? (cadr variable))
         (string-append (symbol->string (caddr variable))
                        "-"
                        (number->string (cadr variable)))
         (symbol->string (cadr variable))))))


;;;SECTION 4.4.4.8
;;;Frames and bindings
(define (make-binding variable value)
  (cons variable value))

(define (binding-variable binding)
  (car binding))

(define (binding-value binding)
  (cdr binding))


(define (binding-in-frame variable frame)
  (assoc variable frame))

(define (extend variable value frame)
  (cons (make-binding variable value) frame))


;;;;From Section 4.1

(define (tagged-list? exp tag)
  (if (pair? exp)
      (eq? (car exp) tag)
      false))

(define (prompt-for-input string)
  (newline) (newline) (display string) (newline))


;;;;Stream support from Chapter 3

(define (stream-map proc s)
  (if (stream-null? s)
      the-empty-stream
      (cons-stream (proc (stream-car s))
                   (stream-map proc (stream-cdr s)))))

(define (stream-for-each proc s)
  (if (stream-null? s)
      'done
      (begin (proc (stream-car s))
             (stream-for-each proc (stream-cdr s)))))

(define (display-stream s)
  (stream-for-each display-line s))
(define (display-line x)
  (newline)
  (display x))

(define (stream-filter pred stream)
  (cond ((stream-null? stream) the-empty-stream)
        ((pred (stream-car stream))
         (cons-stream (stream-car stream)
                      (stream-filter pred
                                     (stream-cdr stream))))
        (else (stream-filter pred (stream-cdr stream)))))

(define (stream-append s1 s2)
  (if (stream-null? s1)
      s2
      (cons-stream (stream-car s1)
                   (stream-append (stream-cdr s1) s2))))

(define (interleave s1 s2)
  (if (stream-null? s1)
      s2
      (cons-stream (stream-car s1)
                   (interleave s2 (stream-cdr s1)))))

;;;;Table support from Chapter 3, Section 3.3.3 (local tables)

(define (make-table)
  (let ((local-table (list '*table*)))
    (define (lookup key-1 key-2)
      (let ((subtable (assoc key-1 (cdr local-table))))
        (if subtable
            (let ((record (assoc key-2 (cdr subtable))))
              (if record
                  (cdr record)
                  false))
            false)))
    (define (insert! key-1 key-2 value)
      (let ((subtable (assoc key-1 (cdr local-table))))
        (if subtable
            (let ((record (assoc key-2 (cdr subtable))))
              (if record
                  (set-cdr! record value)
                  (set-cdr! subtable
                            (cons (cons key-2 value)
                                  (cdr subtable)))))
            (set-cdr! local-table
                      (cons (list key-1
                                  (cons key-2 value))
                            (cdr local-table)))))
      'ok)    
    (define (dispatch m)
      (cond ((eq? m 'lookup-proc) lookup)
            ((eq? m 'insert-proc!) insert!)
            (else (error "Unknown operation -- TABLE" m))))
    dispatch))

;;;; From instructor's manual

(define get '())

(define put '())

(define (initialize-data-base rules-and-assertions)
  (define (deal-out r-and-a rules assertions)
    (cond ((null? r-and-a)
           (set! THE-ASSERTIONS (list->stream assertions))
           (set! THE-RULES (list->stream rules))
           'done)
          (else
           (let ((s (query-syntax-process (car r-and-a))))
             (cond ((rule? s)
                    (store-rule-in-index s)
                    (deal-out (cdr r-and-a)
                              (cons s rules)
                              assertions))
                   (else
                    (store-assertion-in-index s)
                    (deal-out (cdr r-and-a)
                              rules
                              (cons s assertions))))))))
  (let ((operation-table (make-table)))
    (set! get (operation-table 'lookup-proc))
    (set! put (operation-table 'insert-proc!)))
  (put 'and 'qeval conjoin)
  (put 'or 'qeval disjoin)
  (put 'not 'qeval negate)
  (put 'unique 'qeval uniquely-asserted)
  (put 'lisp-value 'qeval lisp-value)
  (put 'always-true 'qeval always-true)
  (deal-out rules-and-assertions '() '())) 

;; Do following to reinit the data base from microshaft-data-base
;;  in Scheme (not in the query driver loop)
;; (initialize-data-base microshaft-data-base)

(define microshaft-data-base
  '(
;; from section 4.4.1
(address (Bitdiddle Ben) (Slumerville (Ridge Road) 10))
(job (Bitdiddle Ben) (computer wizard))
(salary (Bitdiddle Ben) 60000)

(address (Hacker Alyssa P) (Cambridge (Mass Ave) 78))
(job (Hacker Alyssa P) (computer programmer))
(salary (Hacker Alyssa P) 40000)
(supervisor (Hacker Alyssa P) (Bitdiddle Ben))

(address (Fect Cy D) (Cambridge (Ames Street) 3))
(job (Fect Cy D) (computer programmer))
(salary (Fect Cy D) 35000)
(supervisor (Fect Cy D) (Bitdiddle Ben))

(address (Tweakit Lem E) (Boston (Bay State Road) 22))
(job (Tweakit Lem E) (computer technician))
(salary (Tweakit Lem E) 25000)
(supervisor (Tweakit Lem E) (Bitdiddle Ben))

(address (Reasoner Louis) (Slumerville (Pine Tree Road) 80))
(job (Reasoner Louis) (computer programmer trainee))
(salary (Reasoner Louis) 30000)
(supervisor (Reasoner Louis) (Hacker Alyssa P))

(supervisor (Bitdiddle Ben) (Warbucks Oliver))

(address (Warbucks Oliver) (Swellesley (Top Heap Road)))
(job (Warbucks Oliver) (administration big wheel))
(salary (Warbucks Oliver) 150000)

(address (Scrooge Eben) (Weston (Shady Lane) 10))
(job (Scrooge Eben) (accounting chief accountant))
(salary (Scrooge Eben) 75000)
(supervisor (Scrooge Eben) (Warbucks Oliver))

(address (Cratchet Robert) (Allston (N Harvard Street) 16))
(job (Cratchet Robert) (accounting scrivener))
(salary (Cratchet Robert) 18000)
(supervisor (Cratchet Robert) (Scrooge Eben))

(address (Aull DeWitt) (Slumerville (Onion Square) 5))
(job (Aull DeWitt) (administration secretary))
(salary (Aull DeWitt) 25000)
(supervisor (Aull DeWitt) (Warbucks Oliver))

(can-do-job (computer wizard) (computer programmer))
(can-do-job (computer wizard) (computer technician))

(can-do-job (computer programmer)
            (computer programmer trainee))

(can-do-job (administration secretary)
            (administration big wheel))

(rule (lives-near ?person-1 ?person-2)
      (and (address ?person-1 (?town . ?rest-1))
           (address ?person-2 (?town . ?rest-2))
           (not (same ?person-1 ?person-2))))

(rule (same ?x ?x))

(rule (wheel ?person)
      (and (supervisor ?middle-manager ?person)
           (supervisor ?x ?middle-manager)))

(rule (outranked-by ?staff-person ?boss)
      (or (supervisor ?staff-person ?boss)
          (and (supervisor ?staff-person ?middle-manager)
               (outranked-by ?middle-manager ?boss))))
))