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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 parse(self):
 """ Parses the CSS contents and returns the cleaned CSS as a string :returns: The cleaned CSS :rtype: str """

        # Build the HTML tree
        self.tree = self._build_tree(self.html_contents)

        # Parse the CSS contents
        self.stylesheet = self.parser.parse_stylesheet(self.css_contents)

        # Get the cleaned CSS contents
        self.cleaned_css = self._clean_css()