Applied to a list of lists, union returns a list of all the elements of them,
in some unspecified order, with no repetitions. It can be considered as the
union of the family of `sets'.
FAILURE CONDITIONS
Never fails.
EXAMPLE
# unions [[1;2]; [2;2;2;]; [2;3;4;5]];;
val it : int list = [1; 2; 3; 4; 5]