  This paper explores the usefulness of a technique from software engineering,
namely code instrumentation, for the development of large-scale natural
language grammars. Information about the usage of grammar rules in test
sentences is used to detect untested rules, redundant test sentences, and
likely causes of overgeneration. Results show that less than half of a
large-coverage grammar for German is actually tested by two large testsuites,
and that 10-30% of testing time is redundant. The methodology applied can be
seen as a re-use of grammar writing knowledge for testsuite compilation.
