A time series is a serially indexed collection of items.
Time series data have a natural temporal ordering.
This makes time series analysis distinct from other common data analysis problems that have no natural ordering of the observations.
A time series model will generally reflect the fact that observations close together in time will be more closely related than observations further apart.
In addition, a time series model will often make use of the natural one-way ordering of time so that
values for a given period will be expressed as deriving in some way from past values, rather than from future values.
This implementation represents each datum and its timestamp as a pair of value and index often called time.
The entries (pairs) are sorted in ascending order by the timestamps, c.f., TimeSeries.Entry.
A TimeSeries is immutable.