The Gem Counter's Ledger

In the vault of Luma, the royal jeweler keeps a short registry of gem marks. Each character in jewels is a registered gem mark, and each character in stones is one stone found in today's shipment. A mark is case-sensitive, so a and A are different marks.

Return how many stones carry a mark that appears in the registry. A repeated mark in jewels still describes one registered mark, while every occurrence in stones is counted separately.

Use a set of registered marks, then inspect the shipment one character at a time. Do not sort either string or count only distinct shipment marks.

Examples
Input: ["aA","aAAbbbb"]
Output: 3
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The Gem Counter's Ledger

In the vault of Luma, the royal jeweler keeps a short registry of gem marks. Each character in `jewels` is a registered gem mark, and each character in `stones` is one stone found in today's shipment. A mark is case-sensitive, so `a` and `A` are different marks.