code: public Datatype.Builder mutateNestedClasses(Consumer<? super List<Excerpt>> mutator) {
      if (nestedClasses instanceof ImmutableList) {
        nestedClasses = new ArrayList<>(nestedClasses);
      }
      // If addNestedClasses is overridden, this method will be updated to delegate to it
      mutator.accept(nestedClasses);
      return (Datatype.Builder) this;
    }
docs: Applies {@code mutator} to the list to be returned from {@link Datatype#getNestedClasses()}.
  
  <p>This method mutates the list in-place. {@code mutator} is a void consumer, so any value
  returned from a lambda will be ignored. Take care not to call pure functions, like {@link
  Collection#stream()}.
  
  @return this {@code Builder} object
  @throws NullPointerException if {@code mutator} is null