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I have just tried the above solution, and while Textastic seems to recognize the Taskpaper format (I can choose it under Syntax Definition, but it is also detected by Taskpaper on files with the extension .taskpaper), it does not seem to apply any highlighting. I've used the current version of the tmbundle, so things may have changed in the last two years.

So two questions, the first one of a more general character:

1. How do I debug/errorfind on a tmbundle in Textastic?

2. Does anyone work with the current (or any) taskpaper tmbundle in Textastic?