code: function() {
      this.jsonInit({
        "message0": Blockly.Msg.LOOKS_GOFORWARDBACKWARDLAYERS,
        "args0": [
          {
            "type": "field_dropdown",
            "name": "FORWARD_BACKWARD",
            "options": [
              [Blockly.Msg.LOOKS_GOFORWARDBACKWARDLAYERS_FORWARD, 'forward'],
              [Blockly.Msg.LOOKS_GOFORWARDBACKWARDLAYERS_BACKWARD, 'backward']
            ]
          },
          {
            "type": "input_value",
            "name": "NUM"
          }
        ],
        "category": Blockly.Categories.looks,
        "extensions": ["colours_looks", "shape_statement"]
      });
    }
docs: "Go forward/backward [Number] Layers" Block.
  @this Blockly.Block