Euclidean distance standardised against a vector of standard
    deviations per coordinate.

    ..math::
        D(x, y) = \sqrt{\sum_i \frac{(x_i - y_i)**2}{v_i}}

def standardised_euclidean(x, y, sigma=_mock_ones):
    """Euclidean distance standardised against a vector of standard
    deviations per coordinate.

    ..math::
        D(x, y) = \sqrt{\sum_i \frac{(x_i - y_i)**2}{v_i}}
    """
    result = 0.0
    for i in range(x.shape[0]):
        result += ((x[i] - y[i]) ** 2) / sigma[i]

    return np.sqrt(result)