Challenge 1: Find Shared Tags — Possible Solution
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127.0.0.1:6379> SINTER post:1:tags post:2:tags
1) "redis"
2) "databases"

WHY THIS WORKS AS AN ANSWER
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SINTER takes any number of set keys and returns only the members
present in ALL of them — the intersection. post:1:tags contains
{redis, databases, nosql} and post:2:tags contains {redis, sql,
databases}. Comparing them: "redis" appears in both, "databases"
appears in both, but "nosql" only appears in post:1:tags and "sql"
only appears in post:2:tags — so only redis and databases survive the
intersection.

This is exactly the chapter's own likes:redis / likes:databases
example, just applied to a tags scenario instead of a "who likes both"
scenario — the same SINTER mechanics answer both "which users like
both things" and "which tags do both posts share," since both are
really the same question: what do these two sets have in common.