docs: def _resolved_url(self):
          """Return a URL that properly combines the base_url and a possibly relative
          resource url"""
  
          if not self.url:
              return None
  
          u = parse_app_url(self.url)
  
          if u.scheme == 'index':
              u = u.resolve()
  
          if u.scheme != 'file':
              # Hopefully means the URL is http, https, ftp, etc.
  
              return u
          elif u.resource_format == 'ipynb':
  
              # This shouldn't be a special case, but ...
              t = self.doc.package_url.inner.join_dir(self.url)
              t = t.as_type(type(u))
              t.fragment = u.fragment
  
              return t
  
          elif u.proto == 'metatab':
  
              u = self.expanded_url
  
              return u.get_resource().get_target()
  
          elif u.proto == 'metapack':
  
              u = self.expanded_url
  
              if u.resource:
                  return u.resource.resolved_url.get_resource().get_target()
              else:
                  return u
  
          if u.scheme == 'file':
  
              return self.expanded_url
  
          elif False:
  
  
              assert isinstance(self.doc.package_url, MetapackPackageUrl), (type(self.doc.package_url), self.doc.package_url)
  
              try:
                  t = self.doc.package_url.resolve_url(self.url) # Why are we doing this?
  
                  # Also a hack
                  t.scheme_extension = parse_app_url(self.url).scheme_extension
  
                  # Another Hack!
                  try:
                      if not any(t.fragment) and any(u.fragment):
                          t.fragment = u.fragment
                  except TypeError:
                      if not t.fragment and u.fragment:
                          t.fragment = u.fragment
  
  
                  # Yet more hack!
                  t = parse_app_url(str(t))
  
                  return t
  
              except ResourceError as e:
                  # This case happens when a filesystem packages has a non-standard metadata name
                  # Total hack
                  raise
  
          else:
              raise ResourceError('Unknown case for url {} '.format(self.url))