a file with extension ".m4" is authomatically converted into an ".html"
with conversions like:

RUmLmDmUmLmRDm -> <a href="../symmetric/details.html#RUmLmDmUmLmRDm">RUmLmDmUmLmRDm<img src="../images/011-013-101-103-110-112-121-211s_t.png"></a>

S3 -> <a href="nourse.html">S-3</a>


------------------- sample .m4 page: -------------------------------------
<ul>
<li>Start from RUmLmDmUmLmRDm
<li>One of the faces of the closed cube is a flap. Push it <b>inside</b>
the cube
<li>Lift the newly formed flap into the final shape
</ul>