f {
  %t = $mid($1,2)
  %d = $asc($1) - $asc(%t)
  if (!$2 || %d = $2) && %t && $abs(%d) = 1 %r $calc(1+$g(%t,%d))
}
g %r $f($1,$2)
h %r $reduce($1)
reduce {
  %r = return
  %i = $f($1) + 1
  if ($1) %r $+($left($1,1),$iif(%i > 2,-),$h($mid($1,$iif(%i > 2,%i,2))))
}