Function contact_residuals 

pub fn contact_residuals(
    kind: &Kind,
    a: &Geometry,
    pa: Option<&Pose<D>>,
    b: &Geometry,
    pb: Option<&Pose<D>>,
    feet: &[D],
) -> GeopResult<Vec<D>>

The residuals of a constraint of kind that keeps the follower b on a — a point on a curve, a circle or a point touching a face — at the foot point feet (see Kind::feet), where pa and pb put them: the follower’s point less the foot point, three lengths.