VCCH, chapter 4 (Classes), page 51

     A Class_header appearing in the text of a  class  `C'
     is  valid  if  and only if it satisfies either of the
     following two conditions:

        1 There is no header mark of  the  deferred  form,
          and `C' is effective.

        2 There is a Header_mark of the deferred form, and
          `C' is deferred.
