Added set lemmas about difference and union

Added some set lemmas about difference and union that I was missing in my local development.

added 1 commit

• d4e3d4da - Added parentheses for clarity, and explicitly named used variable

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added 1 commit

• 413cdb23 - Use unicode arrow

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added 1 commit

• 2fcd3395 - Simplified lemma and names

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added 1 commit

• ded5e23c - Renamed lemmas as per suggestion

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added 2 commits

• 2f651a42 - Bumped use of renamed lemmas.
• e1d8e672 - Updated CHANGELOG and added change to sed script

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added 1 commit

• 18c0382d - Apply 1 suggestion(s) to 1 file(s)

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added 1 commit

• d06725e5 - Fixed sed script entry

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resolved all threads

Merging. Thanks for the many times we had to go back and forth, @jihgfee!

Actually, I cannot merge. Please rebase to master and squash all commits.

added 105 commits

• d06725e5...c71531e0 - 97 commits from branch iris:master
• e7e1b170 - Added set lemmas about difference and union
• 53e0ecc3 - Added parentheses for clarity, and explicitly named used variable
• 943d5ef9 - Use unicode arrow
• 8c3b8cee - Simplified lemma and names
• da2c9507 - Renamed lemmas as per suggestion
• 38542f8f - Bumped use of renamed lemmas.
• a737ce26 - Updated CHANGELOG and added change to sed script
• 0884d5ca - Fixed sed script entry

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added 1 commit

• b93a066f - Added set lemmas about difference and union

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added 1 commit

• bdf49b49 - Write CHANGELOG in present tense.

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resolved all threads

enabled an automatic merge when the pipeline for bdf49b49 succeeds