(=> (q 'sd)
    (map create-document (find-all-from)))

(define (find-all-from)
  (uniquify
    (map contents (node-set->list (q '(type: at name: from owner: fare))))))

(define (create-document from)
  (create-root from
	       output-dtd
	       (subtree-spec add: '((type sd))
                             children: (list (create-table from)))))

(define (create-table from)
  (create-element 'table
		  '()
		  (cons (create-title from)
			(append	(create-header-rows)
				(map (lambda (to)
				        (create-data-row from to))
				     (find-all-to from))))))

(define (find-all-to from)
  (sort-uniquify
    (map contents
	 (q `(type: at name: to owner: (fare attribute: from ,from))))))
	     
(define (create-element gi attributes children)
  (subtree-spec add: `((type el) (gi ,gi))
	        attributes: attributes
		children: children))

(define (create-data-row from to)
  (create-element row
		  '()
		  (list (create-cell from to 'adult 'single)
			(create-cell from to 'adult 'return)
			(create-cell from to 'child 'single)
			(create-cell from to 'child 'return))))

(define (create-cell from to child return)
  (create-element 'cell
		  '()
		  (string->subtree-spec
		   (contents (q `(fare attribute: from ,from
				       attribute: to ,to
				       attribute: child ,child
				       attribute: return ,return))))))

;; the remaining functions are left as an exercise to the reader