docs: def vlink(s_expnum, s_ccd, s_version, s_ext,
            l_expnum, l_ccd, l_version, l_ext, s_prefix=None, l_prefix=None):
      """make a link between two version of a file.
  
      @param s_expnum:
      @param s_ccd:
      @param s_version:
      @param s_ext:
      @param l_expnum:
      @param l_ccd:
      @param l_version:
      @param l_ext:
      @param s_prefix:
      @param l_prefix:
      @return:
      """
      source_uri = get_uri(s_expnum, ccd=s_ccd, version=s_version, ext=s_ext, prefix=s_prefix)
      link_uri = get_uri(l_expnum, ccd=l_ccd, version=l_version, ext=l_ext, prefix=l_prefix)
  
      return client.link(source_uri, link_uri)