title: How to get the direction of a marking arrow point to the tangent of decoration point on the curve
body: <p>I need to decorate a curve with the center of the arrow at the center of the curve. In <code>tikz</code> this can be achieved by setting the value of <code>pre length</code> equal to the length of the arrow, as in the following example </p>
  
  <pre><code>\draw[decoration={ pre length=8pt, markings, mark=at position 0.5 with {\arrow{Stealth[black,length=8pt]}}}, postaction={decorate}] (-0.5, 0.125) to [out=60, in=120] (0.5, 0.125);
  </code></pre>
  
  <p>However, now the direction of arrow is the tangent of the point where the arrow tip is, which is not symmetric and not nice at all. <a href="https://i.stack.imgur.com/W7p0h.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/W7p0h.png" alt="enter image description here"></a> </p>
  
  <p>Is it possible to get the direction of the arrow the same as the tangent of decoration point  (<code>pos=0.5</code> here) while keeping the center of the arrow at the same position so that the picture looks symmetric ? The following picture is what I need. <a href="https://i.stack.imgur.com/f4Kgp.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/f4Kgp.png" alt="enter image description here"></a></p>