Array2d for h == 0

[aobatact]

• plan1 : prohibit h == 0
  • An assertion ?
• plan2 : make h as NonZeroUsize
  • It is bit noisy but most elegant
• plan3 : support h == 0
  • a: Make heads as dangling pointer if h == 0 (How do we hold a v length if we don't have any heads?) or,
  • b: Make heads as dummy vec and hold v length (This needs some extra branch for all the ops, can we permit this for an unuseful edge case?)
• plan4 : ignore
  • worst decision

I think plan 2 or 3a(return vlen as 0 for h == 0) is better.