'usage: crop <int:left> <int:upper> <int:right> <int:lower>
<image:pic1>
Crop and push a rectangular region from the current image.'

def do_crop(self):

    left = int(self.do_pop())
    upper = int(self.do_pop())
    right = int(self.do_pop())
    lower = int(self.do_pop())
    image = self.do_pop()
    self.push(image.crop((left, upper, right, lower)))
