clip-systems png output?

[ian rashkin]

hi, am a total newbie to lilypond, but way impressed, hope to use it a lot.
interested in outputting clips as png, so trying various combinations, all of 
which are the same result.

file scale.ly: 

\version "2.12.0"  % necessary for upgrading to future LilyPond versions.
#(ly:set-option 'clip-systems #t)

\header{
  %title = "A scale in LilyPond"
}
\score {

        \relative c' {
          c d e f g a bes c d ees8( e) c4 b c~ c1
        }

        \layout {
          clip-regions
          = #(list
              (cons
               (make-rhythmic-location 3 0 4)
               (make-rhythmic-location 4 0 4)))
        }
}

lilypond --png scale.ly 

i get a clipped eps (or so it would appear), and a full ps and png. if i leave 
out the --png, i get a clipped pdf. 

am i doing this wrong, or is it not possible to get a png clip?

thanks, 
ian