unchecked-knowable.scala:18: warning: fruitless type test: a value of type Bippy cannot also be a A1
  /*   warn */ (new Bippy).isInstanceOf[A1]
                                       ^
unchecked-knowable.scala:19: warning: fruitless type test: a value of type Bippy cannot also be a B1
  /*   warn */ (new Bippy).isInstanceOf[B1]
                                       ^
error: No warnings can be incurred under -Xfatal-warnings.
two warnings found
one error found