This section is incomplete.
Seq<T> implements a lazy sequence (often called a "stream") of values of type T. It has a super simple interface for retrieving these values consisting only of the method Maybe<T> next(). As can be seen, it returns a Maybe<T> which is similar to boost::optional, Haskell's Maybe or other similar types that either contain a value or be empty. next successively returns the values in the sequence until the sequence is exhausted after which it will return an empty Maybe to signal that there are no more values.
Seqs are mutable since calling next will modify the Seq by advancing it. They also have value semantics since you can copy them and pass them around like any other value.