$a[0][0] = ['00', '01'];
$a[0][1] = ['10', '11'];
DB<69> x @a[0]
0  ARRAY(0x55bb8ce673d8)
   0  ARRAY(0x55bb8ce5b430)
      0  00
      1  01
   1  ARRAY(0x55bb8d543fb8)
      0  10
      1  11