KGS math club/solution 10 2

Something is an member of P iff it is a vole or is of the form (p,q) where p and q are both already known to be members of P.

(Why a vole? Because Uberdude supplied a hint that the answer begins with the letter V, and a vole seemed as good as anything else. Vole are small rodents, like mice but chubbier.)