$doubleStack = [
                 [
                   'a',
                   'b',
                   'c'
                 ],
                 [
                   'e',
                   '1',
                   '2'
                 ],
                 [
                   '3',
                   '4',
                   '5'
                 ],
                 [
                   '3',
                   'f'
                 ]
               ];