Relay and tunnelling shield: what they have in common

1. Step 1. Relay — is part of — Traffic light. Relay controllers.
2. Step 2. Traffic light — has part — LED. LEDs.
3. Step 3. LED — is part of — Street light. LEDs.
4. Step 4. Street light — is part of — Tunnel. Lighting.
5. Step 5. Tunnel — uses — Tunnelling shield. Shield tunnelling.

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