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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 sgn(x):
 """ Return the sign of x. Return a positive integer if x > 0, 0 if x == 0, and a negative integer if x < 0. Raise ValueError if x is a NaN. This function is equivalent to cmp(x, 0), but more efficient. """

    x = BigFloat._implicit_convert(x)
    if is_nan(x):
        raise ValueError("Cannot take sign of a NaN.")
    return mpfr.mpfr_sgn(x)