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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 hardmask(self):
 """ Mask all lowercase nucleotides with N's """

        p = re.compile("a|c|g|t|n")
        for seq_id in self.fasta_dict.keys():
            self.fasta_dict[seq_id] = p.sub("N", self.fasta_dict[seq_id])
        return self