Simulating a differential equation with discontinuous right hand side

Hi,

How can I simulate a switched system with discontinuous right hand side? Consider a linear system

xdotplus=[1 1; -1 -1]*x + [3;1]*u

and the switching conditions is given by

u={ 0 ; if x1+x2 >0 1 ; if x1+x2 <0}.

Note that u is discontinuous.