DB<1> @a=1..2
 
  DB<2> x @a[0,1]
0  1
1  2
  DB<3> x @a[0,3]
0  1
1  undef
  DB<4> x @a[2,3]
0  undef
1  undef  
  DB<5> @a=()
 
  DB<6> x @a[1,2]
0  undef
1  undef