  @Test
  public void testConcat() {

    testFutures("() ; () = ()", n1.concat(n2), 0);
    testFutures("() ; a = a", n1.concat(a), 1);
    testFutures("a ; () = a", a.concat(n1), 1);
    testFutures("a ; b = (a, b)", a.concat(b), 2);
    testFutures("() ; P = ()", n1.concat(p1), 0);
    testFutures("P ; P = ()", p1.concat(p2), 0);
    testFutures("P ; () = ()", p1.concat(n1), 0);
    testFutures("P ; a = a", p1.concat(a), 1);
    testFutures("a ; P = a", a.concat(p1), 1);
    testFutures("P ; a ; b ; (); P = (a,b)", p1.concat(a).concat(b).concat(n1).concat(p2), 2);
  }