Notes. The lens mass profile is parametrized by the Einstein radius θ_E, and the source surface brightness by the Sérsic radius R, index n, and half-light I. For both cases, the elliptical geometry is described by its center (x, y) and eccentricities e₁, e₂. 𝒰(a, b) denotes a uniform distribution, 𝒩(μ, σ) a normal distribution, and 𝒯 𝒩(μ, σ; a, b) a truncated normal distribution. The value in parentheses is a different prior for S2 eccentricities as is expected to have low elongation. The source distance prior limits z_{S 2} to (2.1,3.2). All positions and radius (x, y, θ_E and R) are expressed in arcseconds, and the positions are relative to the central point of the pixelated image. I is in micro-Jansky per arcsecond squared. A normal prior on e₁ and e₂ is needed to reflect a rotational symmetry (see Gu et al. 2022).