plus(X,0,X).
plus(X,s(Y),s(Z)):- plus(X,Y,Z).
multi(X,0,0).
multi(X,s(Y),Z):- multi(X,Y,T), plus(X,T,Z).
minus(X,0,X).
minus(s(X),s(Y),Z):- minus(X,Y,Z).
div(0,s(Y),0).
div(s(X),s(Y),s(Z)):- minus(X,Y,T),div(T,s(Y),Z).