code: public Datatype.Builder addNestedClasses(Excerpt element) {
      if (nestedClasses instanceof ImmutableList) {
        nestedClasses = new ArrayList<>(nestedClasses);
      }
      nestedClasses.add(Objects.requireNonNull(element));
      return (Datatype.Builder) this;
    }
docs: Adds {@code element} to the list to be returned from {@link Datatype#getNestedClasses()}.
  
  @return this {@code Builder} object
  @throws NullPointerException if {@code element} is null