public static <S, T, A, B> Lens<S, T, A, B> unLiftB(Lens<S, T, A, Maybe<B>> lens) {
        return lens.mapB(Maybe::just);
    }
public static < S , T , A , B > Lens < S , T , A , B > unLiftB ( Lens < S , T , A , Maybe < B > > lens ) { return lens . mapB ( Maybe :: just ) ; }
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