I downloaded the Anet A6 board definitions from GitHub and unzipped it into the Arduino/Hardware folder. (I also left the zip file in the Hardware folder.) I was then able to select the Anet A6 board in the Arduino IDE and burn the Optiboot bootloader to an Anet A6.
Later, I installed some definitions for ESP8266 boards and the Anet A6 boards disappeared, though the board definition folder is still in the Hardware sub-folder. I may have also installed a more recent version of the Arduino IDE, but i am not sure of that.
Has anyone any idea how I can get the Anet A6 boards to reappear?