<ul>
<li>Start from configuration RRmRmULLmLmU oriented such that the upper-right-front tile is
  of type backslash.
<li> Rotate the upper-right-front tile around its right vertical side (together with
  its lower neighbour) against the upper-right lateral tile.
  It becomes a flap, to be lowered in horizontal position.
<li>
  That flap is the internal horizontal tile of the final configuration, moving it
  into position will free another flap that becomes the external horizontal tile.
</ul>