Function orient2d_2i 

pub fn orient2d_2i(
    a: &ImplicitPoint,
    b: &ImplicitPoint,
    c: [f64; 3],
    axis: DropAxis,
) -> Sign

orient2d with TWO implicit points (a,b) and one explicit (c), projected after axis. orient2d = Λ′/(d1·d2) with Λ′ = (λ1_i−d1·c_i)(λ2_j−d2·c_j) − (λ1_j−d1·c_j)(λ2_i−d2·c_i); both denominators are odd → den_signs = [sign(d1), sign(d2)].