Syntax related problem

sir, What syntax we write as exact solution “sig_e” satisfy average of trace(sig_e)=0 condition.

u_e = Expression(("pi*cos(pi*x[1])*sin(pi*x[0])*sin(pi*x[0])*sin(pi*x[1])","-pi*cos(pi*x[0])*sin(pi*x[1])*sin(pi*x[1])*sin(pi*x[0])"), domain=mesh, degree=4)
p_e = Expression(("x[0]*x[1]*(1-x[0])*(1-x[1])"),domain=mesh, degree=4)
sig_e = 2*mu*epsilon(u_e)-p_e*Identity(2)