PL1 + cPL2 + γ_ext: posterior distributions for several lens-mass parameters of the power-law profile (see Sect. 4) and the external shear strength. We show the distributions for candidate models (i.e., models that predict four observable quasar images) in red, and the distribution of the whole MCMC chain is plotted in black. The two contours show the 1σ and 2σ credible regions. The one-dimensional histograms show the marginalized posterior distribution for the selected mass parameters, and the vertical lines mark the 1σ confidence intervals.