anchor.diff:  
   ;;; user preferences
   
  -(defpage /site/preferences-ok "Preferences updated" ()
  -  #H[Email updated successfully])
  +(defpage /site/preferences-ok "Preferences updated" (what)
  +  #H[${what} updated successfully])
   
   (defhandler /site/change-email (email password)
     (flet ((err (e) #/site/preferences?email={email}&error={e}))
  
positive.code:   </form>
  </div>])))
  
  (defun email-address? (str)
    (and str (not (string= str ""))))
  
  
negative.code:           <dd><input class="regin" type="text" name="email" title="new email"
                       value="${(if email email "")}" /></dd>]
            (maybe-show-form-error error "pw" "Wrong password")
            #H[<dt><label for="password">Enter password:</label></dt>
            <dd><input class="regin" type="password" name="password" /></dd>
            <dt /><dd><input type="submit" value="change email" /></dd>
          </dl>
  