[math-fun] x^2=0 but x=1

I am interested in an analytic device to handle the following simple situation. I have a polynomial P in several variables, x,y,z,u, say. It is of degree 2 in each variable. I would like to evaluate the polynomial when x^2 = 0, y^2 = 0, z^2 = 0, u^2 = 0 BUT x=y=z=u=1. Many thanks for any comment, hint, input. Emeric P.S. Sorry, previous message had no subject.