anchor.diff:                  // callback when Cytoscape Web has finished drawing
                   vis.ready(function() {
   
  -                    //Add new comment field to nodes and edges:
  -                   var comment_field = { name: "Comment", type: "string", defValue: null };
  -                   vis.addDataField(comment_field);
  +                    //get all data fields of nodes and edges
  +                    nodesDataSchema = getDataSchema("nodes");
  +                    edgesDataSchema = getDataSchema("edges");
  +
  +                    var comment_field = { name: "Comment", type: "string", defValue: null };
  +                    //Add new comment field to nodes and edges if not existed:
  +                    if (checkDataField("nodes","Comment") == undefined) {
  +                        vis.addDataField("nodes",comment_field);
  +                        nodesDataSchema = getDataSchema("nodes");
  +                    }
  +                    if (checkDataField("edges","Comment") == undefined) {
  +                        vis.addDataField("edges",comment_field);
  +                        edgesDataSchema = getDataSchema("edges");
  +                    }
   
                       // add a listener for when nodes and edges are clicked
                       vis.addListener("click", "nodes", function(event) {
  -
  +                        //Add new comment field to nodes and edges if not existed:
                           handle_click(event,"nodes");
                       })
                       .addListener("click", "edges", function(event) {
  
positive.code:                                                 col2.textContent = variable_value;
                                                  break;
                                              } else {
                                                  if (i == (key_words.length - 1)) {
                                                      var input_text = document.createElement('input');
                                                      createEditableElement(input_text, target, variable_name, variable_value);
                                                      col2.appendChild(input_text);
                                                  }
                                              }
  
negative.code: 
                                  //hide null attributes except 'Comment'
                                  if(variable_value == null){
  
                                     if( variable_name =="Comment"){
                                       table.appendChild(row);
                                     }
  