Pole arm and solar panel: what they have in common

1. Step 1. Pole arm — is part of — Planimeter. A lever with a fixed pole.
2. Step 2. Planimeter — has part — Wheel. A measuring wheel computes the area.
3. Step 3. Wheel — is part of — Lunokhod. Wheels.
4. Step 4. Lunokhod — has part — Solar panel. A solar panel.

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