Fix multiset after `Value.tag`

[Alex-Fischman]

The Value.tag PR passed CI when it shouldn't have. This PR fixes the compiler error currently on main.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #454 will improve performances by 14.08%

_{Comparing Alex-Fischman:fix-multiset (d7536ad) with main (993582f)}

Summary

⚡ 3 improvements ✅ 5 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	Alex-Fischman:fix-multiset	Change
⚡	cykjson	343.2 ms	320.1 ms	+7.23%
⚡	eggcc-extraction	4.3 s	4.1 s	+5.09%
⚡	math-microbenchmark	4.1 s	3.6 s	+14.08%

[Alex-Fischman]

(CodSpeed is just showing the same speedup as on Value.tag.)