sclass _SubModulePrint implements Appendable {
  S prefix;
  bool beginningOfLine = true;
  
  *() {}
  *(S *prefix) {}
  
  public Appendable append(char c) { ret append(str(c)); }
  public Appendable append(CharSequence csq, int start, int end) { ret append(csq.subSequence(start, end)); }
  
  public Appendable append(CharSequence csq) {
    S s = str(csq);
    if (empty(s)) this;
    if (beginningOfLine) print_raw(makePrefix());
    bool nl = s.endsWith("\n");
    if (nl) s = dropLast(s);
    if (nempty(prefix) && contains(s, '\n'))
      s = s.replace("\n", "\n" + prefix);
    print_raw(s);
    if (nl) print_raw("\n");
    beginningOfLine = nl;
    this;
  }
  
  swappable S makePrefix() { ret prefix; }
}

Began life as a copy of #1012700