unbeamed quavers

[Chris Jackson]

I want to use manual beaming to indicate that quavers are beamed by default,
but occasionally some quavers are non-beamed, like so.

\version "1.7.21"

\score {
    <
 \notes \relative c'' {a8 b c b [a] [b] [c d]}
    >
    \paper { }
}

But this produces ugly warnings about the beam having less than two stems.
Are these warnings necessary, or are [ ] not supposed to be used like that,
or should I be setting the automatic beaming properties instead?

--
Chris