*     
 *     s = [ 1, 2, 3 ]           #=> [1, 2, 3]
 *     t = [ 4, 5, 6, [7, 8] ]   #=> [4, 5, 6, [7, 8]]
 *     a = [ s, t, 9, 10 ]       #=> [[1, 2, 3], [4, 5, 6, [7, 8]], 9, 10]
 *     a.flatten                 #=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10
 *     a = [ 1, 2, [3, [4, 5] ] ]
 *     a.flatten(1)              #=> [1, 2, 3, [4, 5]]
 */

In line containing "a.flatten" word, there is no closing bracket ( "]" ).