A citation key references some external material that is part of a bibliografical collection.
Currently, Zettelstore implements this only partially, it is
work in progress.
However, the syntax is: beginning with a left square bracket and followed by an at sign character (
@, U+0040), a the citation key is given.
The key is typically a sequence of letters and digits.
If a comma character (
,, U+002C) or a vertical bar character is given, the following is interpreted as inline elements.
A right square bracket ends the text and the citation key element.