Here's a reference link.

Here's one on the next line.

Here's one with a different URL.

Here's one with a semantic name.

Here's one with an upper case label definition.

Here's one with definition name on the next line.

Here's [one][404] with no definition.

Here's an image:

Here's an implicit one.

Here's an implicit one with an empty link definition.

Here's a multiline one defined on 2 lines.