NotEmptySet type inheritance

[LVMVRQUXL]

Description

Update the implementation of the NotEmptySet type for not inheriting from a type like the following:

data class NotEmptySet<A>(val head: A, val tail: NotEmptySet<A>? = null) {
    val asSet: Set<A> get() = TODO()
    val size: StrictlyPositiveInt get() = TODO()
}

Checklist

• Implement.
• Test.
• Refactor.
• Update Work in progress section in changelog.

[LVMVRQUXL]

The tail parameter should be optional:

data class NotEmptySet<E>(val head: E, val tail: Set<E>? = null)