anchor.diff:     {simple-error} is signaled."
     (check-type name keyword)
     (check-type agent agent)
  -  (with-lock-grabbed (*registry-lock*)
  +  (with-write-lock (*registry*/lock)
       (if #1=(gethash name *registry*)
           (error "~a is already registered." name)
           (setf #1# agent))))
  
positive.code:   "*Description*:
  
     {registered} returns a _list_ of names associated with _agents_."
    (with-lock-grabbed (*registry-lock*)
      (loop for name being the hash-keys of *registry* collect name)))
  
  (defun agent-by-name (name)
    "Return agent by NAME if registered, signal error otherwise."
    (with-lock-grabbed (*registry-lock*)
      (or (gethash name *registry*)
          (error "No such agent: ~a" name))))
  
negative.code:    If the _name_ is not associated with an _agent_ an _error_ of _type_
     {simple-error} is signaled."
    (check-type name keyword)
    (with-write-lock (*registry*/lock)
      (or (remhash name *registry*)
          (error "Not registered: ~a" name))))
  
  