anchor.diff:  ;;;;USER-INTERFACE:    UNIX
   ;;;;DESCRIPTION
   ;;;;    This module exports some list utility functions.
  -;;;;USAGE
  -;;;;
   ;;;;AUTHORS
   ;;;;    <PJB> Pascal J. Bourguignon <pjb@informatimago.com>
   ;;;;MODIFICATIONS
  +;;;;    2012-03-14 <PJB> Added plist-keys.
   ;;;;    2012-02-19 <PJB> Moved HASHED-* functions that work on sequence to
   ;;;;                     COM.INFORMATIMAGO.COMMON-LISP.CESARUM.SEQUENCE.
   ;;;;    2011-04-03 <PJB> Added LIST-LENGTHS.
  
positive.code:            "EQUIVALENCE-CLASSES" "SUBSETS" "COMBINE" "IOTA"
             "MAKE-LIST-OF-RANDOM-NUMBERS" "LIST-INSERT-SEPARATOR"
             "NSPLIT-LIST-ON-INDICATOR" "NSPLIT-LIST" "DEEPEST-REC" "DEEPEST" "DEPTH"
             "FLATTEN" "LIST-TRIM" "TRANSPOSE" "AGET" "MEMQ" "PLIST-REMOVE" "PLIST-GET"
             "PLIST-PUT" "HASHED-INTERSECTION"
             ;; "HASHED-REMOVE-DUPLICATES" moved to COM.INFORMATIMAGO.COMMON-LISP.CESARUM.SEQUENCE
             "ENSURE-LIST" "PROPER-LIST-P" "LIST-LENGTHS"
  
negative.code:       :finally (return (values i i 0)))))
  
  
  (defun plist-keys (plist)
    "Returns a list of the properties in PLIST."
    (loop :for (key) :on plist :by (function cddr) :collect key))
  
  
  (defun plist-put (plist prop value)
    "
  