was curious about this while looking through:
github.com/alicevision/geogram/blob/05b3ef4edf0b87b5e52ff7f93a520e0e98ab3951/src/lib/geogram/basic/quaternion.h#L67
- was useful to overcome submodule error for 'pybind11' in github.com/tdegeus/pybind11_examples
- the command mentioned in Edit 2, 'git submodule sync' is crucial