(module ancestors-struct mzscheme
  
  (require (planet "inference.ss" ("williams" "inference.plt")))
  
  ;;; Example Inference Model
  
  (define-struct parents (name mother father) (make-inspector))
  
  (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))
      #(struct:parents ?name ?mother ?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 (make-parents 'penelope 'jessica 'jeremy))
     (assert (make-parents 'jessica 'mary-elizabeth 'homer))
     (assert (make-parents 'jeremy 'jenny 'steven))
     (assert (make-parents 'steven 'loree 'john))
     (assert (make-parents 'loree #f 'jason))
     (assert (make-parents 'homer 'stephanie #f))
     (start-inference)))
  
  (find-ancestors)

  )