\turn_over_horizontal allows to flip over the sheet of paper
over the horizontal axis. This means that all points located in the
upper part of the paper will be drawn symmetrically in the bottom in
the next step (same for bottom). You have to keep in mind that this
operator modifies the coordinates of all the previously defined
vertices. The official symbol (rounded arrow) of turn over is also
printed in the current step.
This operator should only appear
after a step block.
\turn_over_horizontal;