API and typewriter: what they have in common

1. Step 1. API — is a kind of — Software. An interface between programs.
2. Step 2. Software — includes — Operating system. A program.
3. Step 3. Operating system — uses — Graphical user interface. The graphical shells of an OS.
4. Step 4. Graphical user interface — evolved from — Command line. The graphical interface replaced the text one.
5. Step 5. Command line — evolved from — Typewriter. The teletype was a printing terminal.

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