Pulse oximeter and typewriter: what they have in common

1. Step 1. Pulse oximeter — is part of — Smartwatch. A heart-rate and SpO₂ sensor.
2. Step 2. Smartwatch — has part — Touch layer (capacitive). A touchscreen.
3. Step 3. Touch layer (capacitive) — is part of — Self-checkout. A touchscreen.
4. Step 4. Self-checkout — evolved from — Cash register. A till.
5. Step 5. Cash register — has part — Keyboard. Sums were entered with keys.
6. Step 6. Keyboard — is part of — Typewriter. The keys drive levers with type.

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