anchor.diff:  
   The player's date of birth, if it is known.
   
  -@item AGE
  -
  -The  player's age,  if it  is known.  Note that  the reader  method will
  -compute the age from the date-of-birth when one is present.
  +@item AGE%
  +
  +The  player's age,  if  it  is known.  Note  that  the related  function
  +`PLAYER-AGE'  will  compute the  age  from  the date-of-birth  when  one
  +is present.
   
   For sanity-checking purposes only, the age  is clamped to 0…130. This is
   mostly to detect the not-uncommon  data-entry error of someone supplying
   a  two-digit year-of-birth  that  is later  interpreted  as their  being
   1,900+ years old.
  +
  +Do  not call  `PLAYER-AGE%'  directly, please.  Use `PLAYER-AGE',  which
  +falls back upon computing from the player's date-of-birth.
   
   @end itemize"
     (id (binary<-uuid (uuid:make-v4-uuid))
  
positive.code:   (surname nil :type (or null string))
    (full-name nil :type (or null string))
    (date-of-birth nil :type (or null local-time:date))
    (age nil :type (or null (real 0 (130)))))
  
  (defun legal-age (date-of-birth &optional (reference-date (local-time:now)))
    "The age of  a person born on DATE-OF-BIRTH, as  of REFERENCE-DATE (or
  
negative.code: 
  ;;; Creatures, Players.
  
  (datafly:defmodel player
    "A  structure representing  a  player  who is  a  human  being in  the
   real world.
  