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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 cat(expr, others=None, sep=None, na_rep=None):
 """ Concatenate strings in sequence with given separator :param expr: :param others: other sequences :param sep: string or None, default None :param na_rep: string or None default None, if None, NA in the sequence are ignored :return: """


    if others is not None:
        from .strings import _cat as cat_str

        return cat_str(expr, others, sep=sep, na_rep=na_rep)

    return _cat(expr, sep=sep, na_rep=na_rep)