code: public <A, B> Mutator<T> bi(TriConsumer<? super T, A, B> consumer, A a, B b) {
      consumer.accept(t, a, b);
      return this;
    }
docs: Passes the underlying value to a consumer, with two arguments.
  Usage:
  <pre>
  Map&lt;String, String&gt; map = mutate(new HashMap&lt;String, String&gt;())
  .nil(Map::put, "a", "A")
  .nil(Map::put, "b", "B")
  .nil(Map::put, "c", "C")
  .map(Collections::unmodifiableMap)
  .get();
  </pre>
  
  @param consumer typically a method reference for T
  @param a        first argument
  @param b        second argument
  @param <A>      type of a
  @param <B>      type of b
  @return this