gh-509: new inv_triangle_number popped up in another PR

[Saransh-cpp]

Description

My bad. #523 introduced some occurrences of inv_triangle_number which went unnoticed. Prime example of why you should make your PRs "up-to-date" before merging.

Refs: #509

Changelog entry

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[paddyroddy]

Lesson learned, it's a useful feature!