;;; added as answer to exercise 5.1.1 - 3: SPR *list* --> SPR max-one-list
sign := *top* & 
[ HEAD pos,
  SPR max-one-list,
  COMPS *list* ].

pos := *top*.
noun := pos.
verb := pos.
det := pos.
;;; added as answer to exercise 5.1.1 - 2
prep := pos.

phrase := sign &
[ ARGS *list* ].

word := sign &
[ ORTH string ].

;;;;;;;;;;;;;;;;;;;;;;;

string := *top*.

*list* := *top*.

;;; modified as answer to exercise 5.1.1 - 3
*ne-list* := *list* &
[ FIRST *top*,
  REST *top* ].

;;; added as answer to exercise 5.1.1 - 3
one-plus-list := *ne-list* &
[ REST *ne-list*].

;;; added as answer to exercise 5.1.1 - 3
max-one-list := *list*.

;;; added as answer to exercise 5.1.1 - 3
one-list := max-one-list & *ne-list* &
[ REST *null* ].

;;; modified as answer to exercise 5.1.1 - 3
*null* := max-one-list.