Most Arduino boards contain an LED and a load resistor connected between pin 13 and ground; a convenient feature for many simple tests.[16] The previous code would not be seen by a standard C++ compiler as a valid program, so when the user clicks the "Upload to I/O board" button in the IDE, a copy of the code is written to a temporary file with an extra include header at the top and a very simplemain() function at the bottom, to make it a valid C++ program.