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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 is_url(text):
 """ Check if the given text looks like a URL. """

    if text is None:
        return False
    text = text.lower()
    return text.startswith('http://') or text.startswith('https://') or \
        text.startswith('urn:') or text.startswith('file://')