With your code (many thanks, its really save us time), we were able to create a beta version : https://dl.dropboxusercontent.com/u/13064071/plantuml.jar
This version support the following syntax:
:i := i + 1]
:i > 5}
First, we could not use the \ as ending character, because this character is used to span lines ( http://plantuml.sourceforge.net/qa/?qa=208/ability-to-span-lines )
This version is not the final/official release for this syntax. We provide it to let people play with it : this is usually a good way to have several new ideas/suggestions.
About the suggestion to replace the first character ":" by something else, like in
or (more verbose):
It might be a good idea, but you have to keep in mind that the plantuml parser has to guess which diagrams has to be produced (sequence? class? usecase?) and we have to pay attention to not allow ambiguous syntax that may be interpreted be several kind of diagrams.
So do use this beta version, and we are very open to other suggestions, like this one (that we did not understand, sorry...):
<>:i > 5;