Assume a point-like source located at (0''.07, 0''.0), which is lensed by a SIS lens
with critical radius b = 0''.7 and external shear with amplitude gamma = 0.1 and
direction theta_gamma = 0, and calculate:

a) the image positions and magnifications

b) the critical curves (plot them together with the image positions and discuss
the situation in the lens plane)

c) the caustics (plot them together with the source position and discuss the
situation in the source plane)