Add additional mutation to > (resp. <)

[nfrankel]

Most tests miss testing the boundary. Mutation testing can help a lot with this. The current code misses the change from > to >= and < and <=.

[nfrankel]

The failing test checks for a typo on a source file I didn't touch.

[sourcefrog]

Hi, thanks for the patch.

I think this is worth considering, but we ought to check whether it generates too many low-value hits. Have you tried it on a large tree.

[nfrankel]

I leave it up to you, but this mutation is part of established mutation testing tools.

https://pitest.org/quickstart/mutators/#CONDITIONALS_BOUNDARY

[sourcefrog]

I know, but it would also be useful to know that you tried it on some tree and it found a certain fraction of interesting or uninteresting misses.

[nfrankel]

I didn't try it on a regular codebase. I just wanted to test this specific use-case, which is the one where Mutation Testing shines, and found it lacking.

[sourcefrog]

Next steps:

• Aside from the typo warning, which I fixed in main, there are some tests failing due to this generating new mutants that are not expected by the tests
• After the tests pass, mutants test the mutants tree and see if anything is caught, and whether they seem like interesting findings
• book/src/mutants.md should be updated to describe this
• Add it to NEWS

[nfrankel]

I did my best, but I'm stuck on one of the tests in error. I can't understand where I should set the expectation.