static TextEdit createTextEdit(S text, S newText) {
  S a = text, b = newText;
  int i = lCommonPrefix(a, b);
  if (i == l(a) && i == l(b)) null; // Strings are equal
  a = substring(a, i);
  b = substring(b, i);
  int j = lCommonSuffix(a, b);
  a = substring(a, 0, l(a)-j);
  b = substring(b, 0, l(b)-j);
  
  // Now differing part remains in a and b;
  // i and j give length of unchanged beginning+end
  if (empty(a) && empty(b)) null;
  
  TextEdit edit = nuWithParams(TextEdit, 
    startIndex := i, endIndex := i+l(a), text := b);

  // Verify!
  assertEquals(newText, replayTextEdit(edit, text));
  
  ret edit;
}