el : int -> 'a list -> 'a
Synopsis: Extracts a specified element from a list.
el i [x1,...,xn] returns xi. Note that the elements are numbered starting
from 1, not 0.
el if the integer argument is less than 1 or greater than the
length of the list.
- el 3 [1,2,7,1];
> val it = 7 : int
Lib, hd, tl, index.