[coord] Fix minor bug for exact negative z-axis as reference vector x0

6 jobs for master in 3 minutes and 48 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Lint
passed #531609
docker
Lint

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  Unittest
passed #531610
docker
Unittest27

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passed #531611
docker
Unittest35

00:00:49

 
  Coverage
passed #531612
docker
Coverage

00:00:32

92.0%
 
  Deploy
passed #531613
docker
pages

00:00:40

passed pages:deploy

00:00:02