Stuff

** foo.cpp **
#include "external.h"

void foo() {
    int sample = digitalRead(1);
    Serial.print(sample);
}

** bar.cpp **
#include "external.h"

void bar() {
    int sample = analogRead(2);
    Serial.print(sample);
}


** external.h **
#include <(elusive libraries)>
void foo();
void bar();

** arduino sketch **
#include "external.h"

// rest of sketch follows here