code: public <A> Mutator<T> un(BiConsumer<? super T, A> consumer, A a) {
      consumer.accept(t, a);
      return this;
    }
docs: Passes the underlying value to a consumer, with an argument.
  Usage:
  <pre>
  List&lt;String&gt; list = mutate(new ArrayList&lt;String&gt;())
  .nil(List::add, "a")
  .nil(List::add, "b")
  .nil(List::add, "c")
  .nil(List::remove, "b")
  .map(Collections::unmodifiableList)
  .get();
  </pre>
  
  @param consumer typically a method reference for T
  @param a        an argument
  @param <A>      argument type
  @return this