There can be no cycles in the subtype relation between .QL classes.

Example error and correction

class Y extends X, Type {
  string toString() { result="Y" }
}

class X extends Y {
  string toString() { result="X" }
}

Notice the cyclic dependency between X and Y. This is usually a serious design problem, and you probably need to rethink the class hierarchy of your query completely. A common solution is to make a single class that encompasses all the classes in the cycle. Here that would be something like

class XY  extends Type {
  string toString() { result = "X" or result="Y" }
}