Maxima: how to replace cos(phi) with x

Question

I have an expression cos(phi) + sin(phi). How can I convert this expression it to x+y with maxima, assuming that cos(phi) is x, and sin(phi) is y?

Answer 1

expr: cos(phi) + sin(phi);
subst([cos(phi)=x, sin(phi)=y], expr);

Answer 2

Or just

(%i1) cos(phi) + sin(phi), cos(phi) = x, sin(phi) = y;
(%o1) y + x