  This paper explores the usefulness of a technique from software engineering,
code instrumentation, for the development of large-scale natural language
grammars. Information about the usage of grammar rules in test and corpus
sentences is used to improve grammar and testsuite, as well as adapting a
grammar to a specific genre. 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. This methodology applied can be
seen as a re-use of grammar writing knowledge for testsuite compilation.
