The path is tiled with two types of trapezoidal units: a green one that turns right and a blue one that turns left.

Choose the action you want from the menu on the left.

"Switch" switches an existing tile right to left or vice versa when you click it.

When you choose "Right" or "Left", a new tile is added when you click; the new tile is added along the path at a location closest to where you clicked, and a locator is shown at the click.

"X" deletes a tile when you click it.

A collision occurs when a vertex of a trapezoid enters a gray rectangle; then such a vertex becomes red.