New to arduino here. How do I debounce multiple buttons?

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Here is how those two routines can be used to read a button:

#define KEY0 0 //key0 to pin 0
#define LED1 1 //led1 to pin 1

void loop(void) {
  if (key_read(KEY0) == KEY_PRESSED) digitalWrite(LED1, !digitalRead(LED1)); //if key0 is pressed, flip led1
}

You can replace key_read() with key_read2() and they would function the same - the actually debouncing is totally transparent to the calling application.