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* Issue 154: ``pkg_resources`` will now cache the zip manifests rather than
re-processing the same file from disk multiple times, but only if the
environment variable ``PKG_RESOURCES_CACHE_ZIP_MANIFESTS`` is set. Clients
that package many modules in the same zip file will see some improvement
in startup time by enabling this feature. This feature is not enabled by
default because it causes a substantial increase in memory usage.