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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _validate_pixel_density(self):
 """ Validate image pixel density See `spec <https://ocr-d.github.io/mets#pixel-density-of-images-must-be-explicit-and-high-enough>`_. """

        for f in [f for f in self.mets.find_files() if f.mimetype.startswith('image/')]:
            if not f.local_filename and not self.download:
                self.report.add_notice("Won't download remote image <%s>" % f.url)
                continue
            exif = self.workspace.resolve_image_exif(f.url)
            for k in ['xResolution', 'yResolution']:
                v = exif.__dict__.get(k)
                if v is None or v <= 72:
                    self.report.add_error("Image %s: %s (%s pixels per %s) is too low" % (f.ID, k, v, exif.resolutionUnit))