\turn_over_vertical allows to filp over the sheet of paper over
the vertical axis. This means that all points located in the right
part of the paper will be drawn symmetrically in the left in the next
step (same for left). 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_vertical;