class C1 extends Object { }
class C2 extends Object { }
class C3 extends C1 { }
main {
  C1 c1;
  C2 c2;
  C3 c3;
  null==null; //OK
  null==c3;  //OK
  c3==null; //OK
  c1==c3;  //OK because one side is a subtype of the other
  c2==c3; //error: type mismatch with == operator
          //because neither side of the == operator has a type that 
          //is a subtype of the other side's type
  c1==c2; //also a type mismatch with == operator
}