Reserved key for streamline seed point?

With MRtrix3 there's been a few requests over the years to have, for streamlines propagated bidirectionally from the seed point, knowledge of which vertex in the output streamline corresponds to the seed location. See e.g. tckmap -output_seeds option (which was at the time the easiest hack to satisfy demands).

In the context of this format, it's a DPS of either a uintX index or a 3.floatXX, depending on whether the seed point itself or an index in the output streamline is provided (theoretically it's possible that in some implementation the precise seed location won't actually be included in the output streamline; in MRtrix3 we guarantee that it's included, but I'm thinking of the most general case).

Primarily the question is whether or not such information should have some form of reserved keyword so that all implementations know where to write it in order to be trivially accessible / interpretable at read time.

Start by reviewing the issue's discussion of bidirectional streamlines and the linked MRtrix3 tckmap documentation, especially the output_seeds option. Define whether the format should reserve a keyword and whether its value represents a seed vertex index or a precise seed coordinate; done means the representation and interoperability expectations are agreed.