cd(var(I)) = var(I).

cd(app(var(I),T)) = app(var(I),cd(T)).

cd(app(app(U,V),T)) = app(cd(app(U,V)),cd(T)).

cd(app(abs(S),T)) = subst(cd(S),0,cd(T)).

cd(abs(S)) = abs(cd(S)).

precedence([cd,subst]).