Parameter degeneracies from the GravLens Monte-Carlo fitting with a single point-like source. x_s and y_s are the position of the source (in arcsec), with respect to image A, and f_s its flux normalized to that of image A. x_l and y_l are the positions of the center of the adopted softened isothermal ellipsoidal model (in arcsec), e is the ellipticity, and θ the position angle (in degrees). ℳ is the mass scale and s the softening length (see Eq. (B.5)), both in arcsec.