Ventilator and tunnelling shield: what they have in common

1. Step 1. Ventilator — has part — Valve. Valves.
2. Step 2. Valve — is part of — Gas lamp. A gas supply valve.
3. Step 3. Gas lamp — evolved into — Street light. Gas lamps.
4. Step 4. Street light — is part of — Tunnel. Lighting.
5. Step 5. Tunnel — uses — Tunnelling shield. Shield tunnelling.

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