Rotate a list of points by an angle alfa (radians) around pivotal point pb.
    Intended for modifying the control points of trigonometric functions.

def rot_points(pnts, pb, alfa):
    """Rotate a list of points by an angle alfa (radians) around pivotal point pb.
    Intended for modifying the control points of trigonometric functions.
    """
    points = []                   # rotated points
    calfa = math.cos(alfa)
    salfa = math.sin(alfa)
    for p in pnts:
        s = p - pb
        r = abs(s)
        if r > 0: s /= r
        np = (s.x * calfa - s.y * salfa, s.y * calfa + s.x * salfa)
        points.append(pb + fitz.Point(np)*r)
    return points