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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 apply_filter(x, filter=None, axis=0):
 """Apply a filter to an array."""

    x = _as_array(x)
    if x.shape[axis] == 0:
        return x
    b, a = filter
    return signal.filtfilt(b, a, x, axis=axis)