code: def renderShortUsername(ctx, username):
      
      if username is None:
          return ''
      req = inevow.IRequest(ctx)
      localpart, domain = username.split('@')
      host = req.getHeader('Host').split(':')[0]
      if host == domain or host.endswith("." + domain):
          username = localpart
      return ctx.tag[username]
docs: Render a potentially shortened version of the user's login identifier,
      depending on how the user is viewing it.  For example, if bob@example.com
      is viewing http://example.com/private/, then render 'bob'.  If bob instead
      signed up with only his email address (bob@hotmail.com), and is viewing a
      page at example.com, then render the full address, 'bob@hotmail.com'.
  
      @param ctx: a L{WovenContext} which has remembered IRequest.
  
      @param username: a string of the form localpart@domain.
  
      @return: a L{Tag}, the given context's tag, with the appropriate username
      appended to it.