slice sampler with constraints

Dear all,

I have a function

 logPDF=@(x) logpost_1(x, y, lam0, gam0);
    x1 = slicesample(gam,kk,'logPDF',logPDF);

where this function is under the constraint x+lam0<0.999

Is there a way of imposing this restriction on the above function?

Best