Music notation and fitness tracker: what they have in common

1. Step 1. Music notation — is used in — Player piano. The rolls encode notes.
2. Step 2. Player piano — is made of — Paper. The program is a perforated paper roll.
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 — Pulse oximeter. Red and infrared LEDs.
6. Step 6. Pulse oximeter — is part of — Fitness tracker. An optical heart-rate sensor.

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