Consider the Euler's angle sequence ϕ1-θ-ϕ2, where ϕ is a rotation about the most recent x-axis and θ is a rotation about the most recent y-axis. Derive an expression for the body's angular velocity vector in the body frame (p, q, r) in terms of the rotational speeds ̇ϕ1, ̇θ and ̇ϕ2.