% file:  grammar-e.p
%        recognizer for X-bar verb phrases
%        - elimination of left recursion
%        - role of entities stored in database (used/1)

% ---------- base productions ----------

:- dynamic used/1.

sentence --> np(nom), vp, ['.'].
sentence --> [].

% np --> def_article, adj_list, noun.
np(nom) --> [she].
np(obj) --> [the], [space], [shuttle].

vp --> vbar.


% elimination of left-recursion
% vbar --> verb, vcomp.
% vbar --> vbar, vcomp.
vbar --> verb, vcomp(obj).
vbar --> verb, vcomp(obj), lvbar.

lvbar --> vcomp(X), { X \== obj }.
lvbar -->
          (
             { used(a), used(b), write_ln('retracting all for used/1'),
               retractall(used(_));
               true
             }
          ),
          vcomp(X),
          { X \== obj },
          lvbar, { ! }.

vcomp(obj) --> np(obj).
vcomp(modifier) --> pp(X), { \+used(X), assert(used(X)) }.
% vcomp(modifier) --> pp(X).
vcomp(modifier) --> [].

pp(a) --> [at], [the], [airfield].
pp(b) --> [on], [a], [friday].

verb --> [landed].