Fix lookahead #73

[chtenb]

Fix #73

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Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)

[jamesdbrock]

This looks good and your test shows that it does what you want.

But are you sure you want this? Did you read my comments in #73 ? I think there is a reason why Megaparsec and Attoparsec don't backtrack lookAhead on failure. Is it common to use lookAhead in the left branch of an alternative?

Here's some discussion about the usage of lookAhead. https://markkarpov.com/tutorial/megaparsec.html#notfollowedby-and-lookahead

[chtenb]

Did you read my comments in #73 ?

For some reason that notification slipped past me. I've replied there.

[chtenb]

Closing per discussion in #73