anchor.diff:  
     const prevEventIndex = () => {
       const n = param.nowIndex;
  -    if (n === null || n === firstIndex()) {
  +    const firstIndex = 1;
  +    if (n === null || n === firstIndex) {
         return null;
       } else {
         return n - 1;
  
positive.code:   };
  
    //-----------------------------------------
    // refresh [<] avatar [>]
    //-----------------------------------------
  
    const refreshAvatarControls = () => {
  
negative.code: 
    const nextEventIndex = () => {
      const n = param.nowIndex;
      const lastIndex = param.events.length - 1;
      if (n === null || n === lastIndex) {
        return null;
      } else {
        return n + 1;
      }
    };
  
    // - - - - - - - - - - - - - - - - - - - - - - - -
  
  