This equation emerges from so called quasi-classical solutions to the quantum YBE, in which the R -matrix admits an asymptotic expansion in terms of an expansion parameter ℏ ℏ , R ℏ ℏ = I + ℏ ℏ r + O ( ℏ ℏ 2 ) . The classical YBE then comes from reading off the ℏ ℏ 2 coefficient of the quantum YBE (and the equation trivially holds at orders ℏ ℏ 0 , ℏ ℏ ).