??stabilizer parabolic in so(2,3) of "event in 2+1 conformal compactification of flat space-time" ... as rectangular rather than triangular; pretty easy to remember because the translations of flat 2+1 space-time form an abelian subalgebra rather than a heisenberg one ... ??but then what _about_ the triangular one, involving the heisenberg?? ...??any similarly nice conceptual description of the semi-direct product in this case ??