erratum: logic error on p.125

[S Sridhar <sridhar%tekchips%tektronix.csnet@CSNET-RELAY.ARPA>]

On p.125 about the middle of the page in the ribcage-lookup function
 in the scope of the second do:
  
  REPLACE (v (cdar r) (cdr v))) BY (v (cadar r) (cdr v)))
  [the cdar is replaced by cadar]