dup /CropBox pget dup {exch pop} if systemdict /UseCropBox known and {
    /CropBox exch /CropBox pget pop
  } {
    /MediaBox exch get_media_box
  } ifelse
  % stack: [<prevproc>] /Crop|MediaBox <Crop|Media Box>


--------------------

  dup /CropBox pget dup {exch pop} if systemdict /UseCropBox known and {
    /CropBox exch /CropBox pget pop
  } { 
    dup /BleedBox pget dup {exch pop} if systemdict /UseBleedBox known and {
    /BleedBox exch /BleedBox pget pop
    } {
    /MediaBox exch get_media_box
    } ifelse
  } ifelse
  % stack: [<prevproc>] /Crop|Media|BleedBox <Crop|Media|BleedBox>