I'm looking forward function, that works like that $\mathbb{N}^{n+1} \rightarrow \mathbb N$:
$f(y, x_1, x_2, \dots ,x_n)=x_y$
We use projection $\Pi^n_k$, but I need something with "dynamic" size and parameters. I need it for writing primitive version of summation of n numbers. I have seen this function in some proofs like http://www.proofwiki.org/wiki/Bounded_Summation_is_Primitive_Recursive , but I haven't seen it's definition
Thank you