Is there an empty polymorphic variant type? Since the syntax [>] means a polymorphic variant that can have any constructors, it would seem natural for [<] to mean a polymorphic variant that can have no constructors (hence is necessarily empty), but it doesn’t seem to be accepted. The syntax  for an “exact” empty polymorphic variant would conflict with empty lists, but I don’t see a problem with [<].
One reason I think this would be useful is that it would be a subtype of all other polymorphic variant types.
Note also that empty polymorphic variants are supported by the compiler, there is just no syntax for them, partially because they are mostly useless. However, it is perfectly possible to define an empty polymorphic variant ppx. For instance, this test is the compiler testsuite: ocaml/typecore_empty_polyvariant_error.ml at trunk · ocaml/ocaml · GitHub tests an error path that only happens with empty polymorphic variants.
Considering ordinary empty types as a subtype of any type is wrong to me; I’m glad that isn’t the case. But since polymorphic variant types already have subtyping relations, I think it would make sense for them to include an empty version.