Constant YIELD_TOLERANCE 

const YIELD_TOLERANCE: SolveTolerance;

The yield solve’s convergence, tighter than the shared default: a price is near 100, so the default’s relative residual of 1e-10 is 1e-8 per 100, which a yield read to 1e-12 cannot rest on — it was met only because Newton overshoots its own criterion. Converged when the price is within 1e-12 per 100 (a hundred times f64 resolution there), which pins the yield to about 1e-14 through a slope of a few hundred per 100, or when a step has stopped moving the yield at 1e-14. One Newton step more at most.