set_to_str.nim

proc complement(s: set[char]): string =
  var last = '\0'
  result = ""
  for elem in '\0'..'\255':
    if elem notin s:
      if result.len == 0:
        result.add elem
        result.add elem
      elif pred(elem) == last:
        result[^1] = elem # patch last interval
      else:
        result.add elem
        result.add elem
      last = elem
  echo result.len