I have written the notToBeAnInstanceOf() implementation as this

fun <T> Expect<T>.notToBeAnInstanceOf(expected: T): Expect<T> = _logicAppend { toBe(expected) }

I have followed the tobe function as in anyExceptions.kt

fun <T> Expect<T>.toEqual(expected: T): Expect<T> = _logicAppend { toBe(expected) }

please read the description in the issue. Have you written your tests, does it what you want? I am sure this will anwser your question as well