anchor.diff:    };
   
     //-----------------------------------------
  -  // refresh [<] event [>]
  -  //-----------------------------------------
  -  // [<] is enabled if there is an event to the lhs of param.nowIndex
  -  // [>] is enabled if there is an event to the rhs of param.nowIndex
  +  // refresh [<] event-index [>]
  +  //-----------------------------------------
  +  // [<] is enabled if there is an event before param.wasIndex
  +  // [>] is enabled if there is an event after  param.nowIndex
     //-----------------------------------------
   
     const refreshEventControls = () => {
  
positive.code:   };
  
    //-----------------------------------------
    // refresh [<] avatar [>]
    //-----------------------------------------
  
    const refreshAvatarControls = () => {
  
negative.code: 
    const prevEventIndex = () => {
      const n = param.nowIndex;
      const firstIndex = 1;
      if (n === null || n === firstIndex) {
        return null;
      } else {
        return n - 1;
  