def calc_a(dert__):
    """
    Compute vector representation of angle of gradient by normalizing (dy, dx).
    Numpy-broadcasted, first dimension of dert__ is a list of parameters: g, dy, dx
    Example
    -------
    >>> dert1 = np.array([0, 5, 3, 4])
    >>> a1 = calc_a(dert1)
    >>> print(a1)
    array([0.6, 0.8])
    >>> # 45 degrees angle
    >>> dert2 = np.array([0, 450**0.5, 15, 15])
    >>> a2 = calc_a(dert2)
    >>> print(a2)
    array([0.70710678, 0.70710678])
    >>> print(np.degrees(np.arctan2(*a2)))
    45.0
    >>> # -30 (or 330) degrees angle
    >>> dert3 = np.array([0, 10, -5, 75**0.5])
    >>> a3 = calc_a(dert3)
    >>> print(a3)
    array([-0.5      ,  0.8660254])
    >>> print(np.rad2deg(np.arctan2(*a3)))
    -29.999999999999996
    """
    return dert__[[2, 3]] / dert__[1]