Gas burner and programmable logic controller: what they have in common

1. Step 1. Gas burner — is part of — Gas lamp. A coal-gas burner.
2. Step 2. Gas lamp — uses — Coal gas. Coal gas.
3. Step 3. Coal gas — is used in — Traffic light. A gas lamp lit the signal.
4. Step 4. Traffic light — has part — Relay. Relay controllers.
5. Step 5. Relay — evolved into — Programmable logic controller. It replaced relay cabinets.

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