@Test
  void debug() {
    assertExpressional(PersonCriteria.person.bestFriend.value().hobby.with(h -> h.is("a").is("b")),
            "call op=AND",
            "  call op=EQUAL path=bestFriend.hobby constant=a",
            "  call op=EQUAL path=bestFriend.hobby constant=b");
  }
Tests that expression is built correctly by "serializing" it to string.
It is easier to debug expression when it is presented as human-readable tree
criteria/common/test/org/immutables/criteria/ExpressionAsStringTest.java