code: public static <T,U,R,RR extends R> BiFunction<T,U,R> constant(final RR r)
      {
          return (t, u) -> r;
      }
docs: Creates a {@link BiFunction} that always returns the
  given value.
  
  @param <T> The first argument type of the {@link BiFunction}
  @param <U> The second argument type of the {@link BiFunction}
  @param <R> The value type of the {@link BiFunction}
  @param <RR> A subtype of the value type
  @param r The value
  @return The {@link BiFunction}