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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 default_dtype(field=None):
 """Return the default data type of this class for a given field. Parameters field : `Field`, optional Set of numbers to be represented by a data type. Currently supported : `RealNumbers`, `ComplexNumbers` The default ``None`` means `RealNumbers` Returns ------- dtype : `numpy.dtype` Numpy data type specifier. The returned defaults are: ``RealNumbers()`` : ``np.dtype('float64')`` ``ComplexNumbers()`` : ``np.dtype('complex128')`` """

        if field is None or field == RealNumbers():
            return np.dtype('float64')
        elif field == ComplexNumbers():
            return np.dtype('complex128')
        else:
            raise ValueError('no default data type defined for field {}'
                             ''.format(field))