exp(V)       --> term(V1), exp1(V1,V).

exp1(V1,V)   --> ['+'], exp(V2), { V is V1 + V2 }.
exp1(V1,V)   --> ['-'], exp(V2), { V is V1 - V2 }.
exp1(V,V)    --> [].

term(V)      --> factor(V1), term1(V1,V).

term1(V1,V)  --> ['*'], term(V2), { V is V1 * V2 }.
term1(V1,V)  --> ['/'], term(V2), { V is V1 / V2 }.
term1(V,V)   --> [].

factor(X)    --> [X], { number(X) }.
factor(V)    --> ['('], exp(V), [')'].