Multi-stream: pivot a joined feed into a region-by-quarter grid

A pivot is a nested groupingBy plus the full column list taken from the data. Without that column list a region that missed a quarter would print a short row, and the grid would stop lining up.

• Multi-stream: a record with a mutable component is a broken Map key
• Multi-stream: an unmodifiable view is not a snapshot of the feed
• Multi-stream: toMap's merge function runs only on a collision
• Multi-stream: nothing runs until the terminal operation
• Multi-stream: indexing the other side costs memory a sorted merge does not
• Multi-stream: UNORDERED is a promise you make, not one you receive

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Published 2026-09-20

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