private fun <A, B, C> Gen<A>.bind(other: Gen<B>, fn: (A, B) -> C): Arb<C> {
   val arb = when (this) {
      is Arb -> this
      is Exhaustive -> this.toArb()
   }

    return arb.flatMap { a ->
      when (other) {
         is Arb -> other.map { fn(a, it) }
         is Exhaustive -> other.toArb().map { fn(a, it) }
      }
   }
}