Get an element

The n-th element of a list l of size s is addressed by l[n] where n is in [0..s - 1] or [1..s]. The equivalent prefixed function is at, which takes as argument a list and an integer n.
at returns the element of the list of index n.
Input :

[0,1,2][1]

or

at([0,1,2],1)

Output :

1