Moving a polynomial from one ring to another

I'd like to find the number of real roots of a polynomial f obtained as a resultant using mp_res_mat(). To do this, I write:
LIB "solve.lib";
LIB "rootsur.lib";
ring r = (0,x),(y),dp;
poly p1 = 2*(1+x^2)^2*(1+y^2)*(x^5-x+y)-y^4*x*(1+y^2)-2*x^3*(1+x^2)^2;
poly p2 = 2*(1+y^2)^2*(1+x^2)*(y^5-y-x)-x^4*y*(1+x^2)-2*y^3*(1+y^2)^2;
ideal i = p1,p2;
poly f = det(mp_res_mat(i));

I would now like to invoke nrroots(f), but x is a parameter in the ring r, not a variable, so this doesn't work. How can I take f to a new ring s = 0,x,dp and then invoke nrroots(f)?

Thanks in advance! :)

Use imap:
imap maps variables and paramaters of the source ring to the variables and paramters with the same name in the current ring.

Perfect! Thank you, I'm very grateful for your help.