I don't think I have an example, but the main feature to use is the paste action. The paste action will draw another object onto itself where the objects overlap.
So basically place a paster object on the layout.
Change it's hotspot to centered in the properties panel.
Position it over the triangle so that no matter which angle it faces the paster will still cover the arrow completely.
Remember the height of the paster. For this example let's say it's 200.
In events do this:
--- paster: clear to (0,0,0,0)
--- paster: set height to 200
--- Sprite: set visible
--- paster: paste Sprite
--- paster: set invisible
--- paster: set height to 200/2
And that's about it. I hid the triangle Sprite after drawing it to the paster since we don't want it to draw again.