Thumbnail image linking to full version

1. [![alt1](thumb1.img)](full1.img)
   - [![alt2](thumb2.img)]
   - [![alt2](thumb2.img)][]
   - [![alt3]](full3.img)
   - [![alt3][]](full3.img)
   - [![alt4]]
   - [![alt4][]][]
5. [![alt5][thumb]][full]

  [alt2]: full2.img
  [![alt2](thumb2.img)]: alternate2.img
  [alt3]: thumb3.img
  [alt4]: picture4.img
  [thumb]: thumb5.img
  [full]: full5.img


All implementations get #1 right and most also #5. Many can do something useful with #3, some also with #2, few with #4. The reference implementations are only supporting #1, though. Is this really intentional?

(Pandoc tries to be overly smart about the fake .img extension.)

Here is an alternative test case (on dingus) for variant 4.

- [![alt]]
- [![alt]][]
- [![alt]][alt]
- [![alt][]]
- [![alt][alt]]
- [![alt][]][]
- [![alt][alt]][]
- [![alt][]][alt]
- [![alt][alt]][alt]

[alt]: 1.png
[![alt]]: 2.png
[![alt][]]: 3.png
[![alt][alt]]: 4.png

The reason Commonmark does not support any of the shortcut syntax variants is that a link label must not contain any square brackets (except backslash-escaped ones), even though a link text can if it forms a valid image reference. This is a bit unfortunate and arguably inconsistent.