assign(assign(ID, equals, EXPR, semicolon)) --> id(ID), equals, expr(EXPR), semicolon.

assign([E|Es]) --> assign(E), assign(Es).

semicolon --> [';'].

equals --> [=].

id(id(ID)) --> [ID], { atom(ID) }.

expr(expr(TERM, ADD_SUB, EXPR)) --> term(TERM), operator(ADD_SUB), expr(EXPR).

expr(expr(TERM)) --> term(TERM).

term(term(FACTOR, MULT_DIV, TERM)) --> factor(FACTOR), operator(MULT_DIV), term(TERM).

term(term(FACTOR)) --> factor(FACTOR).

factor([factor, LEFT, EXPR, RIGHT]) --> paren(LEFT), expr(EXPR), paren(RIGHT).

factor(factor(INT)) --> value(INT).

operator(ADD_SUB) --> [ADD_SUB], { memberchk(ADD_SUB, [+, -]) }.

operator(MULT_DIV) --> [MULT_DIV], { memberchk(MULT_DIV, [*, /]) }.

paren(PARA) --> [PARA], { memberchk(PARA, ['(', ')']) }.

value(int(INT)) --> [INT], {integer(INT)}.

value(symbol(SYMBOL)) --> [SYMBOL], {atom(SYMBOL)}.


parse(ParseTree, Program, []):-
    write(Program),
    assign(ParseTree,Program,[]),
    write('Parser output:\n'), write(ParseTree), write('\n').