Remove the greedily contracted pair by index

[mtfishman]

Summary

The greedy contraction order dropped the pair it had just contracted with filter(∉((a1, a2)), ...), which removes every argument equal to that pair rather than the two chosen. A product with three equal arguments lost one of them and materialized to a scalar. The pair is now chosen and removed by index.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.20%. Comparing base (641dac2) to head (d6652f1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #251      +/-   ##
==========================================
+ Coverage   75.18%   75.20%   +0.02%     
==========================================
  Files          29       29              
  Lines        1797     1799       +2     
==========================================
+ Hits         1351     1353       +2     
  Misses        446      446              

Flag	Coverage Δ
docs	26.20% <0.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.