split_after : int -> 'a list -> 'a list * 'a list
STRUCTURE
Lib
SYNOPSIS
Breaks a list in two at a specified index.
DESCRIPTION
An invocation split_after k [x1,...,xk,...xn] returns the pair ([x1,...,xk], [xk+1,...,xn]). If k is 0, then split_after k l returns ([],l). Similarly, split_after (length l) l returns (l,[]).

FAILURE
If k is negative, or longer than the length of the list.
EXAMPLE
- split_after 2 [1,2,3,4,5]
> val it = ([1, 2], [3, 4, 5]) : int list * int list

- split_after 0 [1,2,3,4,5];
> val it = ([], [1, 2, 3, 4, 5]) : int list * int list

- split_after 5 [1,2,3,4,5];
> val it = ([1, 2, 3, 4, 5], []) : int list * int list

- split_after 6 [1,2,3,4,5];
! Uncaught exception: 
! HOL_ERR

- split_after 0 ([]:int list);
> val it = ([], []) : int list * int list

SEEALSO
partition, pluck
HOL  Kananaskis-10