// Main function that user can call
static long stupidFib_collab(ICollab collab, int n) {
  ret collab_computeLong2(collab, out -> stupidFib_collab(collab, n, out));
}

// Work function used internally
svoid stupidFib_collab(ICollab collab, int n, LongConsumer continuation) {
  // switch to serial if small enough work unit
  if (n < 24) continuation.accept(stupidFib(n));
  
  // recursion base
  if (n < 3) ret with continuation.accept(1L);
  
  // recursion step
  var collector = longPairCollector((a, b) -> continuation.accept(a+b));
  collab.addWork(-> stupidFib_collab(collab, n-1, collector.setterForA()));
  collab.addWork(-> stupidFib_collab(collab, n-2, collector.setterForB()));
}