code: public static <S, T, A, B> Lens<S, T, A, B> unLiftB(Lens<S, T, A, Maybe<B>> lens) {
          return lens.mapB(Maybe::just);
      }
docs: Given a lens with <code>B</code> lifted into {@link Maybe}, flatten <code>B</code> back down.
  
  @param lens the lens
  @param <S>  the type of the "larger" value for reading
  @param <T>  the type of the "larger" value for putting
  @param <A>  the type of the "smaller" value that is read
  @param <B>  the type of the "smaller" update value
  @return the lens with B flattened