gistfile1.kt

public interface ImmutableCollection<out E>: Collection<E> {
    fun <B: Builder<in E>, R> invokeOnBuilder(f: (B) -> R) : R

    public interface Builder<E> {
        fun add(e: E): Builder<E>
        fun build(): ImmutableCollection<E>
    }
}


fun <E1, E: E1> ImmutableCollection<E>.add(e: E1): ImmutableCollection<E1> {
    return this.invokeOnBuilder { builder: ImmutableCollection.Builder<E1> -> builder.add(e).build() }
}