reduce return $left($regsubex($1_,/(.)(?=.)/g,$°(\1,\2,%d)),-1)
° {
  if $calc($asc($1)-$asc($2)) = %s && $v1 isin 1-1 {
    %d =
    if ($3) return -
  }
  else {
    %d = 1
    %s = $v1
    return $1
  }
}