fun start_position() -> ???

Returns the position of the first element in self, or end_position() if self is empty.

fun end_position() -> ???

Returns the "past the end" position in self, that is, the position immediately after the last element in self.

fun position(after i: ???) -> ???

Returns the position immediately after i.

subscript(_ i: ???): ??? { let }

Accesses the element at position i.