Neither the cusp test nor the "ghost images" test from Bozza et.al. 2018 seem to be working for triple lenses. I must be doing something wrong when computing the derivatives of $f(z)$.
A related issue is that as it is currently implemented, the ghost images tests checks that
$$c_g(|I(z_1)| + |I(z_2)|)<\delta$$
where $I$ is an indicator function that evaluates to true if the condition is satisfied, rather than
$$c_g(|I(z_1)|)<\delta\quad \text{and} \quad c_g(|I(z_2)|)<\delta$$
The prohibited regions in the source plane don't look right with the latter condition.
Only way to fix these issues is to re-derive all the math from scratch and check that it all makes sense.
Neither the cusp test nor the "ghost images" test from Bozza et.al. 2018 seem to be working for triple lenses. I must be doing something wrong when computing the derivatives of $f(z)$.
A related issue is that as it is currently implemented, the ghost images tests checks that $$c_g(|I(z_1)| + |I(z_2)|)<\delta$$ where $I$ is an indicator function that evaluates to true if the condition is satisfied, rather than $$c_g(|I(z_1)|)<\delta\quad \text{and} \quad c_g(|I(z_2)|)<\delta$$ The prohibited regions in the source plane don't look right with the latter condition.
Only way to fix these issues is to re-derive all the math from scratch and check that it all makes sense.