PPG TODO
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* fix documentation

* error reporting
  - filename/pos on ppg error
  - add usage message on errors
  - dropping nonexistent symbol

* installation (.jar file)
* option for "base include dir"? (eq. to -I)

* removing "includes"?


In progress
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Complete
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* check multiple level of .ppg inheritance
* dropping (non)terminals
* bin/ppg script
* find harmony with Unix-style path names
* Makefiles
* web/man page
* dropping a list of (non)terminals
* multiple start symbols (new start symbol always required)
* use PPG in building PolyJ
* take care of precedence directives:
  - new precedence rules override all previous ones
  - "precedence;" deletes all precedence rules
