code: function() {
      var container = document.createElement('mutation');
      var isStatement = !this.outputConnection;
      container.setAttribute('statement', isStatement);
      var isAt = this.getInput('AT').type == Blockly.INPUT_VALUE;
      container.setAttribute('at', isAt);
      return container;
    }
docs: Create XML to represent whether the block is a statement or a value.
  Also represent whether there is an 'AT' input.
  @return {Element} XML storage element.
  @this Blockly.Block