def _custom_arrows(sources, targets, angle):
    """
    Create a curved arrow between `source` and `target` as the combination of
    the arc of a circle and a triangle.

    The initial and final angle $lpha$ between the source-target line and
    the arrow is linked to the radius of the circle, $r$ and the distance $d$
    between the points:

    .. math:: r = rac{d}{2 \\cdot 	an(lpha)}

    The beginning and the end of the arc are given through initial and final
    angles, respectively $	heta_1$ and $	heta_2$, which are given with
    respect to the y-axis; This leads to $lpha = 0.5(	heta_1 - 	heta_2)$.
    """
    pass
