Consider a point-like lens with Einstein radius of 1 arcsec located at
coordinate origin, and calculate the images of a point-like background
source located at vec(beta) = (0.3 arcsec, 0.3 arcsec).

Note that the position vectors of the source vec(beta) = (beta_x, beta_y)
and images vec(theta) = (theta_x, theta_y) are always codirectional, and
thus they have the same unit vectors: vec(e) = vec(theta)/theta =
vec(beta)/beta ==> theta_x,y/theta = beta_x,y/beta.