  We carry out numerical experiments in the critical collapse of a spherically
symmetric massless scalar field in 2+1 spacetime dimensions in the presence of
a negative cosmological constant and compare them against a new theoretical
model. We approximate the true critical solution as the $n=4$ Garfinkle
solution, matched at the lightcone to a Vaidya-like solution, and corrected to
leading order for the effect of $\Lambda<0$. This approximation is only $C^3$
at the lightcone and has three growing modes. We {\em conjecture} that
pointwise it is a good approximation to a yet unknown true critical solution
that is analytic with only one growing mode (itself approximated by the top
mode of our amended Garfinkle solution). With this conjecture, we predict a
Ricci-scaling exponent of $\gamma=8/7$ and a mass-scaling exponent of
$\delta=16/23$, compatible with our numerical experiments.
