Allow explicit and implicit numbering of the same tag.

    if t.tagname in by_tagname:

      orig = by_tagname[t.tagname]

      # Allow an explicit tag number to define an implicit tag number

      if orig.tagnum is None:

        orig.tagnum = t.tagnum

      elif t.tagnum is None:

        t.tagnum = orig.tagnum

      if (t.tagnum == orig.tagnum and

          t.description == orig.description):

        # if the name and description are identical, issue a warning