You can think of TiddlyWiki as a database in which the records are tiddlers. A database typically provides a way of discovering which records match a given pattern, and in TiddlyWiki this is done with filters.
A filter is a concise notation for selecting a particular set of tiddlers, known as its output. Whenever TiddlyWiki encounters a filter, it calculates the output. Further work can then be done with just those tiddlers, such as counting or listing them.
That renders as:
A filter output can change as tiddlers are added and deleted in the wiki. TiddlyWiki recalculates on the fly, automatically updating any filter-based counts or lists as well.
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