Retractable roof and hollerith tabulator: what they have in common

1. Step 1. Retractable roof — is part of — Stadium. A retractable roof.
2. Step 2. Stadium — has part — Street light. Floodlights.
3. Step 3. Street light — has part — LED. LEDs.
4. Step 4. LED — is part of — Traffic light. LEDs.
5. Step 5. Traffic light — has part — Relay. Relay controllers.
6. Step 6. Relay — is part of — Hollerith tabulator. Relays controlled the sorting box.

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