While the abstract Numeral inherits only Cat[Numeral,Digits], some of the
concrete NumeralNNN of Numeral inherited everything from CatNNN.
Normally the compiler outputs a warning message when a concrete syntax
contains superflous lincats, but apparently not when they get included
through inheritance...
This does not seem to cause problems in the Haskell run-time system, but the
C run-time system fails to load PGFs with superflous lincats.
This problems shows up when creating application grammars that inherit Numeral.
The Phrasebook is an example of such a grammar.
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