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As of mid September '06 it can build ASTs and tree parsers are working. 
The next ANTLR beta release will include a working Xcode plugin, so you can automatically generate your grammars from within Xcode.
It automatically tracks all the dependencies between the different grammars in your project to build them in the right order.
Common error messages and warnings are picked up for display in Xcode using the new -message-format gnu option@http://www.antlr.org/wiki/display/ANTLR3/Command+line+optionsoption in ANTLR.
There remain some errors and warnings which cannot be picked up yet, due to missing information in ANTLR itself, but this will change in the future.
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