Fixtype of Syntheto type definers.
This is a tagged union type, introduced by fty::deftagsum.
These are the possible definers of Syntheto types; they are the ``right sides'' of a type definition, where the ``left side'' is the name of the type being defined.
The possible definers are type products, type sums, and type subsets. For now we do not allow other types (see type), and in particular we do not allow type synonyms.