There is several ways of creating your own @Formula commands. One easy way is to create a serverside scriptlibrary and create a function that is named @MyCommand there. Include this SSJS library into your XPage / Custom Control and you have added a custom @Formula.
return @Left(data,” “)
And the second way is to create your own extension to the Domino designer as Ulrich Krause has done in this new openntf project.
There is probably serveral more ways of adding new @ commands to XPages, if you know a way make a comment.