- Proof of work was invented to prevent spam. 
- Satoshi used PoW to rate-limit block production.
- Blocks create scarce Satoshis.
- Cashu puts Satoshis into bearer ecash tokens.
- KeyChat (privacy messenger) uses Cashu tokens as stamps to prevent spam.

Full circle  nostr:npub1qg8j6gdwpxlntlxlkew7eu283wzx7hmj32esch42hntdpqdgrslqv024kw.