little tool for tournament cross-tables

establish a fairly fundamental invariant

This commit doesn't guarantee we'll always catch it. But if this invariant is violated, things can get quite difficult to debug. I found in the Lua Carousel fork that all the xpcalls I keep around were actively hindering my ability to notice this invariant being violated.

Created by  akkartik  on November 24, 2023
IPMYE6WTLGB3AWB7VHGSLBLW6PGDX7AEGKFY3KUTXOCQTFQPU6XAC
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