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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 _irregular(singular, plural):
 """ A convenience function to add appropriate rules to plurals and singular for irregular words. :param singular: irregular word in singular form :param plural: irregular word in plural form """

    def caseinsensitive(string):
        return ''.join('[' + char + char.upper() + ']' for char in string)

    if singular[0].upper() == plural[0].upper():
        PLURALS.insert(0, (
            r"(?i)({}){}$".format(singular[0], singular[1:]),
            r'\1' + plural[1:]
        ))
        PLURALS.insert(0, (
            r"(?i)({}){}$".format(plural[0], plural[1:]),
            r'\1' + plural[1:]
        ))
        SINGULARS.insert(0, (
            r"(?i)({}){}$".format(plural[0], plural[1:]),
            r'\1' + singular[1:]
        ))
    else:
        PLURALS.insert(0, (
            r"{}{}$".format(singular[0].upper(),
                            caseinsensitive(singular[1:])),
            plural[0].upper() + plural[1:]
        ))
        PLURALS.insert(0, (
            r"{}{}$".format(singular[0].lower(),
                            caseinsensitive(singular[1:])),
            plural[0].lower() + plural[1:]
        ))
        PLURALS.insert(0, (
            r"{}{}$".format(plural[0].upper(), caseinsensitive(plural[1:])),
            plural[0].upper() + plural[1:]
        ))
        PLURALS.insert(0, (
            r"{}{}$".format(plural[0].lower(), caseinsensitive(plural[1:])),
            plural[0].lower() + plural[1:]
        ))
        SINGULARS.insert(0, (
            r"{}{}$".format(plural[0].upper(), caseinsensitive(plural[1:])),
            singular[0].upper() + singular[1:]
        ))
        SINGULARS.insert(0, (
            r"{}{}$".format(plural[0].lower(), caseinsensitive(plural[1:])),
            singular[0].lower() + singular[1:]
        ))