docs: def post_generate_identifier(request):
      """MNStorage.generateIdentifier(session, scheme[, fragment]) → Identifier."""
      d1_gmn.app.views.assert_db.post_has_mime_parts(request, (('field', 'scheme'),))
      if request.POST['scheme'] != 'UUID':
          raise d1_common.types.exceptions.InvalidRequest(
              0, 'Only the UUID scheme is currently supported'
          )
      fragment = request.POST.get('fragment', None)
      while True:
          pid = (fragment if fragment else '') + uuid.uuid4().hex
          if not d1_gmn.app.models.ScienceObject.objects.filter(pid__did=pid).exists():
              return pid