The Schema Component Constraint: Simple Type Definition Properties
Correct in section 3.14.6 of Structures reads in part:

  All of the following must be true:

  2 All simple type definitions are derived ultimately from the
    ·simple ur-type definition (so· circular definitions are
    disallowed). That is, it is possible to reach a built-in primitive
    datatype or the ·simple ur-type definition· by repeatedly
    following the {base type type definition}.

Since 'derived' is not defined in Datatypes so as to be a reflexive
relation, this wording is problematic.  Possible recastings include:

Take I

    2 All simple type definitions are, or are derived ultimately from,
    the ·simple ur-type definition (so· circular definitions are
    disallowed). That is, it is possible to reach a built-in primitive
    datatype or the ·simple ur-type definition· by following the {base
    type type definition} zero or more times.

Take II

  2 All simple type definitions are, or are derived ultimately from,
    the ·simple ur-type definition (so· circular definitions are
    disallowed). That is, from any ordinary simple type definition it
    is possible to reach a built-in primitive datatype or the ·simple
    ur-type definition· by following the {base type type definition}
    zero or more times.