1. start a new JShell
2. paste the following and press enter:

sealed interface Animal permits Mammal, Bird { }

sealed class Mammal implements Animal
permits Dog, Cat { }

final class Dog extends Mammal { }
final class Cat extends Mammal { }

final class Bird implements Animal { }

3. then go back, add `abstract` and press enter:

Exception in thread "main" java.lang.VerifyError: verifier detected internal inconsistency or security problem
        at jdk.jdi/com.sun.tools.jdi.VirtualMachineImpl.redefineClasses(VirtualMachineImpl.java:401)
        at jdk.jshell/jdk.jshell.execution.JdiExecutionControl.redefine(JdiExecutionControl.java:90)
        at jdk.jshell/jdk.jshell.Unit.doRedefines(Unit.java:312)
        at jdk.jshell/jdk.jshell.Eval.lambda$compileAndLoad$16(Eval.java:1142)
        at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:196)
        at java.base/java.util.AbstractList$RandomAccessSpliterator.forEachRemaining(AbstractList.java:722)