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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 sign(self, entry, signer=None):
 """Adds and sign an entry"""

        if (self.get(entry) is not None):
            return
        if (entry.rrsig is None) and (self.private is not None):
            entry.rrsig = DNSSignatureS(entry.name,
                    _TYPE_RRSIG, _CLASS_IN, entry, self.private, signer)
        self.add(entry)
        if (self.private is not None):
            self.add(entry.rrsig)