Goal: Make sure each library has the catalog of each other library.

This is written for Python3. I have described the input and output in the docstring rather than here in the README file. There are also a few tests in a Pytest directory.

Debug-print lines have been left in the code to assist the writing of further tests in future, but they have been commented out.

1. Further tests: use the output to update contents of libraries and then see if all the libraries's sets of catalogs are complete.
2. Fuller README.

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