In Mathematica 5.0, I'm trying to associate the definition
f[x]^2 + g[x]^2 =1 with f and g.
f[x]^2 + g[x]^2 ^:=1
tries to associate the definition with Power.
I couldn't find anything in Mathematica 5.0 book, which allows me to use two or more iterations of the upvalue operation. Can one do such a thing with Mathematica?