concept Session {}

concept Event {
  new Ref<Session> session;
}

Event > EChange {
  int startIndex, endIndex;
  S text;
}

Event > ECaret {
  int pos;
}  

static S replayEdit(EChange change, S text) {
  if (change == null) ret text;
  ret substring(text, 0, change.startIndex)
    + change.text
    + substring(text, change.endIndex);
}