Vocabulary: http://www.ivoa.net/rdf/ivoasem
Author: elsa.david@obspm.fr
Date: 2025-11-20

Term: ucd-syntax-code
Action: Addition
Label: has UCD syntax code
Description: UCD atoms canot be freely combined.  Rather, each atom has
	a syntax code, which is one of p (only primary position), s (not in
	primary position), q (freely positionable), e (can be followed by a
	spectral atom), c (can be followed by two spectral atoms), or v (can
	be followed by a frame or axis atom).

Rationale:
If we want to model UCDs as an RDF vocabulary, we will have to have some
means to represent the syntax codes as defined in the current UCDList document
https://ivoa.net/documents/UCD1+/.  A discussion leading up to something
like this proposal is found in https://github.com/ivoa-std/Vocabularies/pull/31.

Certain RDF formalisms would let us define the property's domain.  This
would have the advantage that a suitable validator could ensure that we
only assign allowed syntax codes; however, given that changes are
infrequent and well-reviewed, the benefit of that seems too minor to
justify the extra effort of extending Vocabularies in the VO.