Calendar and railway semaphore: what they have in common

1. Step 1. Calendar — includes — Wall calendar. A calendar.
2. Step 2. Wall calendar — is made of — Paper. Calendars were printed on paper.
3. Step 3. Paper — is material for — Billboard. The poster is paper.
4. Step 4. Billboard — has part — LED. LED screens.
5. Step 5. LED — is part of — Traffic light. LEDs.
6. Step 6. Traffic light — evolved from — Railway semaphore. The first traffic light was a railway-style semaphore.

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