A citation key references some external material that is part of a bibliographical 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.