Function trim_station 

pub fn trim_station(start: f64, end: f64, index: usize, count: usize) -> f64

The pcurve parameter of trim station index of count over [start, end]: start + (end − start)·index / count, evaluated in exactly that order.

The comparand is bit identity with the stations the lanes sampled: the prepared-face cache, the self-crossing detector and any caller mirroring a lane rely on the same floats. At index == count this is NOT end to the bit; the generic scan closes a pcurve’s last chord at end itself.