def sqrt(dimensioned_number):
    """Take the square root of a Dn() instance. All of the
    dimension exponents must be divisible by 2.  math.sqrt()
    is called on the numeric part of the Dn().

    :param dimensioned_number: A Dn() instance.
    :returns: A Dn() instance.
    """
    return dimensioned_number.sqrt()