code: function visit(ast, callback) {
              if(callback && typeof callback.visitNode === 'function') {
              	var ret = visitNode(callback, ast);
                  endVisitNode(callback, ast);
                  if(ret === this.BREAK) {
                      return;
                  }
              }
          }
docs: Visit the given AST top-down node -> attributes -> value -> end visit
  @param {Object} dom	The AST to visit
  @param {Function} callback The visitor callback