docs: public static <T> Mutable<T> fromExternal(final Supplier<T> s, final Consumer<T> c) {
          return new Mutable<T>() {
              @Override
              public T get() {
                  return s.get();
              }
  
              @Override
              public Mutable<T> set(final T value) {
                  c.accept(value);
                  return this;
              }
          };
      }