Function face 

pub fn face(
    q: f64,
    star: Star,
    spin: f64,
    direction: Vec3,
    rref: f64,
    pref: f64,
    acc: f64,
) -> Result<(Vec3, Vec3, f64, f64), RocheError>

‘face’ computes the position and orientation of a face on either star in a binary assuming Roche geometry given a direction, a reference radius and a potential.

Arguments:

• q: the mass ratio = M2/M1.
• star: specifies which star, primary or secondary is under consideration.
• spin: ratio of star in questions spin to the orbital frequency
• dirn: the direction (unit) vector from the centre of mass of the secondary to the face in question.
• rref: reference radius. This is a radius large enough to guarantee crossing of the reference potential. See ref_sphere
• pref: reference potential. This defines the precise location of the face.
• acc: location accuracy (units of separation)

Returns:

• pvec position vector of centre of face (position vector in standard binary coordinates), returned
• dvec orientation vector perpendicular to face, returned
• r distance from centre of mass of star, returned
• g magnitude of gravity at face, returned \exception The routine throws exceptions if it cannot bracket the reference potential. This can occur if the reference radius fails to enclose the face in question, or if the face is so deep in the potential that the initial search fails to reach it. Finally if acc is set too low an exception may be thrown if too many binary chops occur. The behaviour at the L1 point is undefined so do not try to call it there.