Call:
ddhazard(formula = survival::Surv(stop, event) ~ group, data = head_neck_cancer, 
    model = "exponential", by = 1, max_T = 20, a_0 = rep(0, 2), 
    Q_0 = diag(1, 2), Q = diag(0.1, 2), order = 1, control = ddhazard_control(eps = 0.1))

'exponential' model fitted with the 'EKF' method in 3 iterations of the EM algorithm.
