%% file:  g0.pl
%% A grammar must specify variables, terminals, and rules.

%% Here is the context-free grammar for a simple grammar
%% of arithmetic expressions using only addition with
%% the only terminals being 0 and 1.
 
%% s -> v + s | v
%% v -> 0 | 1 

%% Variables are stated as facts.  These facts simply state the names of the variables.
%% In this example, the facts state that s and v are variables.
%% These are all in lower case because Prolog thinks of anything beginning
%% in uppercase as one of its own unbound variables.

variable(s).
variable(v).

%% And here are the two terminals in this grammar...

terminal(0).
terminal(1).
terminal(+).

%% Rules are stated as ordered pairs, where the first element is the left-hand-side
%% of a rule and the second argument is the LIST of elements appearing on the right-hand-side
%% of that rule.

rule(s, [v, +, s]).
rule(s, [v]).
rule(v, [0]).
rule(v, [1]).