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Usage of environment variables
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- The test suite uses the LOGGING variable to determine the PEP 282 logging
  level.  This should be a decimal number with the following meaning: 10
  DEBUG, 20 INFO, 30 WARN, 40 ERROR, 50 CRITICAL (default).

- Some ZEO code still uses environment variables, in particular
  ZEO_CACHE_TRACE, ZEO_CLIENT, INSTANCE_HOME, ZEO_SERVER_PID.

- ZOPE_WATCH_CHECKERS used by src/zope/security/checker.py.

- ZOPE_SERVER_SIMULT_MODE used by src/zope/server/serverchannelbase.py.

- ZOPE_SERVER_DEBUG used by src/zope/server/linereceiver/lineserverchannel.py.

- NEW_ZOPE_EXCEPTION_FORMATTER used by src/zope/testing/.

- PYUNIT_EDITOR, EDITOR, EDITOR_REMOTE used by utilities/unittestgui.py.
