@NotNull
 public Document getDocument(@NotNull FragmentSide side) {
   return myDocuments[side.getIndex()];
 }
 public int getStart(@NotNull FragmentSide side) {
   return myStarts[side.getIndex()];
 }
 @Override
 @NotNull
 public ChangeSide getChangeSide(@NotNull FragmentSide side) {
   return mySides[side.getIndex()];
 }