Must accepts a (rune, error) pair and always returns a rune value or raises a panic.

MustFunc wraps a function that returns a (rune, error) pair with [Must] and returns a new function that always returns a rune, RuneEOF, or panics.

Offset describes an offset into a stream at the time of a call to [Reader.Next].

Reader has a useful zero value, but doesn't have a pushback buffer by default.