sclass StringHead is Closeable {
  settable bool readMode;
  settable bool writeMode;
  gettable BufferedReader reader;
  gettable Writer writer;

  *() {}
  *(Reader reader) { reader(reader); }
  *(Writer writer) { writer(writer); }
  
  selfType reader(Reader reader) {
    this.reader = bufferedReader(reader);
    readMode(true);
    this;
  }
  
  selfType writer(Writer writer) { this.writer = writer; writeMode(true); this; }
  
  void finish {}
  
  S readLine() {
    S line = readLineFromReader(reader);
    if (line == null) close();
    ret line;
  }
  
  void writeLine(S line) ctex {
    writer.write(line);
    writer.write('\n');
  }
  
  void exchangeLine(IF0<S> getter, IVF1<S> setter) {
    if (writeMode())
      writeLine(getter!);
    
    if (readMode())
      setter.get(readLine());
  }
  
  close {
    dispose reader;
    dispose writer;
  }
  
  replace ByteIO with StringIO.
  
  void exchange(ByteIO writable) {
    if (writable != null) writable.readWrite(this);
  }
  
  void exchangeAll(Iterable<? extends ByteIO> writables) {  
    if (writables != null)
      for (writable : writables)
        exchange(writable);
  }
}