Automatic block signalling and plough: what they have in common

1. Step 1. Automatic block signalling — has part — Railway semaphore. Signals.
2. Step 2. Railway semaphore — uses — Lever. The semaphore is set by a lever.
3. Step 3. Lever — is used in — Hoe. The handle works as a lever when loosening soil.
4. Step 4. Hoe — evolved into — Plough. The plough mechanized loosening the soil with a hoe.

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