(module ancestors-alist mzscheme
  
  (require (planet "inference.ss" ("williams" "inference.plt")))
  
  ;;; Example Inference Model
  
  (define-ruleset ancestors-ruleset)
  
  (define-rule (initialize ancestors-ruleset)
      (?start <- (start))
    ==>
      (retract ?start)
      (printf "Please enter the first name of a~n")
      (printf "person whose ancestors you would~n")
      (printf "like to find:~n")
      (assert `(request ,(read))))
  
  (define-rule (print-ancestors ancestors-ruleset)
      (?request <- (request ?name))
      (parents ?name (mother . ?mother) (father . ?father))
    ==>
      (retract ?request)
      (when ?mother
        (printf "~a is an ancestor via ~a~n" ?mother ?name)
        (assert `(request ,?mother)))
      (when ?father
        (printf "~a is an ancestor via ~a~n" ?father ?name)
        (assert `(request ,?father))))
  
  (define-rule (remove-request ancestors-ruleset #:priority -100)
      (?request <- (request ?))
    ==>
      (retract ?request))
  
  (define (find-ancestors)
    (with-new-inference-environment
     (activate ancestors-ruleset)
     (assert '(parents penelope (mother . jessica) (father . jeremy)))
     (assert '(parents jessica (mother . mary-elizabeth) (father . homer)))
     (assert '(parents jeremy (mother . jenny) (father . steven)))
     (assert '(parents steven (mother . loree) (father . john)))
     (assert '(parents loree (mother . #f) (father . jason)))
     (assert '(parents homer (mother . stephanie) (father . #f)))
     (start-inference)))
  
  (find-ancestors)

  )