Don't fail to find struct fields in stdlib history

[bobg]

Fixes #27

[github-actions]

Modver result

This report was generated by Modver,
a Go package and command that helps you obey semantic versioning rules in your Go module.

This PR does not require a change in your module’s version number.
(You might still consider bumping the patchlevel anyway.)

[coveralls]

Pull Request Test Coverage Report for Build 20157130158

Details

• 37 of 46 (80.43%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+1.6%) to 70.0%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
expr.go	37	46	80.43%

Files with Coverage Reduction	New Missed Lines	%
expr.go	3	73.65%

Totals
Change from base Build 20157107114:	1.6%
Covered Lines:	889
Relevant Lines:	1270

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💛 - Coveralls