A mark allows to name a point within a zettel.
This is useful if you want to reference some content in a zettel, either with a [link](00001007040310) or with an [inline-mode transclusion](00001007040324).

A mark begins with a left square bracket, followed by an exclamation mark character (“`!`”, U+0021).
Now the optional mark name follows.
It is a (possibly empty) sequence of Unicode letters, Unicode digits, the hyphen-minus character (“`-`”, U+002D), or the low-line character (“`_`”, U+005F).
An optional text to be explicitly marked is introduced with a vertical bar character (“`|`”, U+007C), followed by some [inline-structured elements](00001007040000).
The mark element ends with a right square bracket.

Examples:

* `[!]` is a mark without a name, the empty mark.
* `[!mark]` is a mark with the name “mark”.
* `[!|some text]`is the empty mark with “some text”.
* `[!mark|some text]`is a mark with the name “mark” and with “some text” that is marked.