Constant MERGE_MIN_ANGLE_RAD 

pub const MERGE_MIN_ANGLE_RAD: f64 = _; // 0.017453292519943295f64

Below this half-angle a narrowing part is partly a tube: it merges with a boattail, and a later part with it, by the smaller of the two angles over the larger, the larger taken as at most this, a judgement: 1°.