LED Pin 13 Input or Output? It is not clear to me whether Pin 13, corresponding to the LED, should be driven by an external voltage or not.

From the documentation, 


  LED: 13. There is a built-in LED driven by digital pin 13. When the
  pin is HIGH value, the LED is on, when the pin is LOW, it's off.


However, in the "Blink" example code I am running, the Pin 13 appears to be driven by the ATmega328:

void setup() {
  // initialize digital pin 13 as an output.
  pinMode(13, OUTPUT);
}

// the loop function runs over and over again forever
void loop() {
  digitalWrite(13, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(100);              // wait for a second
  digitalWrite(13, LOW);    // turn the LED off by making the voltage LOW
  delay(2000);              // wait for a second
}


If I now attach a voltage source to Pin 13, will there be contention issues?
Like any other I/O pin, there should be no problem driving this pin (and
thus the builtin LED) from an external source, as long as you make sure
it is in INPUT mode. If the pin is set to OUTPUT you will likely
damage the Arduino.

Warning: If you have the Blink example already loaded in the
Arduino, the pin is in output mode. Be sure to upload another program,
that does not set pin 13 to output, before connecting anything that
could drive pin 13.