docs: def ckm_tree(Vus, Vub, Vcb, gamma):
      r"""CKM matrix in the tree parametrization and standard phase
      convention.
  
      In this parametrization, the parameters are directly measured from
      tree-level $B$ decays. It is thus particularly suited for new physics
      analyses because the tree-level decays should be dominated by the Standard
      Model. This function involves no analytical approximations.
  
      Relation to the standard parametrization:
  
      - $V_{us} = \cos \theta_{13} \sin \theta_{12}$
      - $|V_{ub}| = |\sin \theta_{13}|$
      - $V_{cb} = \cos \theta_{13} \sin \theta_{23}$
      - $\gamma=\delta$
  
      Parameters
      ----------
  
      - `Vus`: CKM matrix element $V_{us}$
      - `Vub`: Absolute value of CKM matrix element $|V_{ub}|$
      - `Vcb`: CKM matrix element $V_{cb}$
      - `gamma`: CKM phase $\gamma=\delta$ in radians
      """
      return np.array([[sqrt(1 - Vub**2)*sqrt(1 - Vus**2/(1 - Vub**2)),
          Vus,
          Vub/exp(1j*gamma)],
          [-((sqrt(1 - Vcb**2/(1 - Vub**2))*Vus)/sqrt(1 - Vub**2)) - (Vub*exp(1j*gamma)*Vcb*sqrt(1 - Vus**2/(1 - Vub**2)))/sqrt(1 - Vub**2),
          -((Vub*exp(1j*gamma)*Vcb*Vus)/(1 - Vub**2)) + sqrt(1 - Vcb**2/(1 - Vub**2))*sqrt(1 - Vus**2/(1 - Vub**2)),
          Vcb],
          [(Vcb*Vus)/(1 - Vub**2) - Vub*exp(1j*gamma)*sqrt(1 - Vcb**2/(1 - Vub**2))*sqrt(1 - Vus**2/(1 - Vub**2)),
          -((Vub*exp(1j*gamma)*sqrt(1 - Vcb**2/(1 - Vub**2))*Vus)/sqrt(1 - Vub**2)) - (Vcb*sqrt(1 - Vus**2/(1 - Vub**2)))/sqrt(1 - Vub**2),
          sqrt(1 - Vub**2)*sqrt(1 - Vcb**2/(1 - Vub**2))]])