[5th_bike]

You're getting there. The x is not a "ground" point. It is the spot where the one wire to the bulb connects to the two wires from the white-circle sides of the two diodes.
The other wire from the bulb goes to ground.

Just for clarity, note in my "line drawing" the diode on the left points in the opposite direction as the diode on the right. They both only let current flow to the 'x', and not through the other diode. The white circles mark the 'downstream' side of the diodes.