Left: initial prior coverage of the targeted observation (black line), obtained with 20 000 spectra: twice the number used in the subsequent round. The range of the initial prior has been expanded to ensure a similar number of training sample spectra have total number of counts below and above the targeted observation (this ensures the observation to be centered). Right: prior coverage in round 2, based on 10 000 simulations. The allowed parameter space has shrunk considerably. This prior proposal could be used as input for running BXA, significantly speeding up the inference by reducing the parameter space that needs to be explored.