I'm a little confused by the function subscripts Nigel Cutland uses in his 1980 book "Computability: An Introduction to Recursive Function Theory". In 4.16.2: 

"Let $\pi(x, y) = 2^x(2y+1)-1$. Show that $\pi$ is a computable bijection from $\mathbb{N}^2$ to $\mathbb{N}$, and that the functions $\pi\_1$, $\pi\_2$ such that $\pi(\pi\_1(z), \pi\_2(z)) = z$ for all $z$ are computable."

Do $\pi\_1$ and $\pi\_2$ refer to $\pi(1,1)$ and $\pi(2,2)?$ I understand proving the bijection (the first part of the prompt) but the exact meaning of those subscripts is not clear to me.