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.