Primer

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- Factor out generator function. (cbc)

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- Refactor functions into module. (cbc)
- Refactor main namespace in tests. (cbc)
- Added VERSION.txt. (cbc)
- Added download_url to setup. (cbc)

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- Make installed tests runnable as script. (cbc)

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- Make reduce readable. HAFD. (cbc)

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- Single version for Python 2/3. (cbc)

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- Add sieve function. (cbc)
- Rename test methods. (cbc)