@Test
public void testPushPropagator() {

    Propagator b3Propagator = new B3Propagator();
    PropagatorStack propagatorStack = new PropagatorStack(Builtin.HTTP_HEADERS);
    assertEquals(propagatorStack.pushPropagator(Propagator.HTTP_HEADERS), propagatorStack);
    assertEquals(propagatorStack.pushPropagator(b3Propagator), propagatorStack);
    assertEquals(propagatorStack.propagators.size(), 2);
    assertEquals(propagatorStack.propagators.get(0), Propagator.HTTP_HEADERS);
    assertEquals(propagatorStack.propagators.get(1), b3Propagator);
}