def is_field(x):
    """
    Return whether or not ``x`` is a field.

    Alternatively, one can use ``x in Fields()``.

    EXAMPLES::

        sage: R = PolynomialRing(QQ, 'x')
        sage: F = FractionField(R)
        sage: is_field(F)
        True
    """
    return x.is_field()