unoriented frames (points, circles, and ellipses) are easy to
understand, a few words should be spent illustrating
their <em>oriented</em> variants. Intuitively, an oriented circle
(ellipse) is a circle (ellipse) with a radius representing its
orientation, such as the following:</p>

<div class="figure">
 <img src="../demo/frame_standard.jpg"></img>
 <div class="caption">
  <span class="content">
    The standard oriented frame: a unit circle, centered at the
    origin, with a radius pointing downwards. Th