Constant MAXIMUM_FOLD_ANGLE 

pub const MAXIMUM_FOLD_ANGLE: f64 = std::f64::consts::FRAC_PI_2; // 1.5707963267948966f64

The largest angle the extension’s cross-boundary tangent may turn through before the extension is called a FOLD. π/2 is the sign change: past it the extension has stopped going outwards and is doubling back over the patch it came from, which is never a usable carrier.