flatten : 'a list list -> 'a list
STRUCTURE
Lib
SYNOPSIS
Removes one level of bracketing from a list.
DESCRIPTION
An invocation flatten [[x11,...,x1k1],...,[xn1,...,xnkn]] yields the list [x1,...,x1k1,...,xn1,...,xnkn].
FAILURE
Never fails.
EXAMPLE
- flatten [[1,2,3],[],[4,5]];
> val it = [1, 2, 3, 4, 5] : int list

- flatten ([[[]]] : int list list list);
> val it =  [[]]  : int list list

HOL  Kananaskis-13