code: @Nonnull
    public static <A, B> Function<B, A> constant(final A a) {
      return b -> a;
    }
docs: Returns a function that ignores it's arguments and always returns the
  given value.
  
  @param a   The given value.
  @param <A> The type of the argument which is returned by the returned function.
  @param <B> The type of the returned functions parameter.
  @return The constant function.