From: "Richard Fateman" <fateman at cs.berkeley.edu>
...
I believe Allegro CL uses the specified CLtL2 branch cuts.
I don't know why they differ from CLtL1, or GCL.
Because X3J13 changed the ATAN branch cut between CLtL1 and CLtL2.
See the X3J13 issue writeup at
http://www.lispworks.com/documentation/HyperSpec/Issues/iss192_w.htm
Kahan is mentioned there.