old_contents: function build(outline) {
      
  }
  
  exports.build = build;
new_contents: var endNewLine = 0;
  var startNewLine = 0;
  
  
  function nNewLines(n) {
      var str = '';
      
      for(var i = 0 ; i < n ; i++) str += '\n';
      
      return str;
  }
  
  
  function nTabs(n) {
      var str = '';
      
      for(var i = 0 ; i < n ; i++) str += '\t';
      
      return str;
  }
  
  
  function traverse(root, level) {
      var walls = '', i;
      
      
      if( level !== 0 ) {
          walls += nTabs(level-1)+`${root.key}('${root.value}', function() {\n`;
      }
      
      
      var childWalls = (root.children || []).reduce(function(prev, child, idx) {
          if( idx === 0 ) prev = nNewLines(startNewLine) + prev;
          
          return prev + traverse(child, level+1)+'\n'+nNewLines(endNewLine);
      }, '');
      
      
      walls += childWalls === '' ? nTabs(level)+'// your codes here\n' : childWalls;
      
      
      if( level !== 0 ) {
          walls += nTabs(level-1)+'});';
      }
      
      return walls;
  }
  
  function build(outline) {
      return traverse(outline, 0);
  }
  
  exports.build = build;
subject: Add traverse function.