Constant MIN_DIVISIBLE_MAGNITUDE 

pub const MIN_DIVISIBLE_MAGNITUDE: f64 = 1.491_668_146_240_041_3e-154;

The magnitude below which a reactance, an impedance, or a tap ratio stops being a number a builder can divide by.

It is f64::MIN_POSITIVE.sqrt(): the square of anything smaller underflows to zero, and the reciprocal is above 1e153, which annihilates every real branch sharing its diagonal. Each builder compares a magnitude against it — |x|, hypot(r, x), the tap — never r² + x², which is a square. Per unit reactances run from 1e-6 to 10, so it rejects poison and nothing else.