An instant of type `a` is the following data:
— A set of labels
— A timestamp
— An element of `a`
The set of labels identifies which timeseries the instant belongs to.
We sometimes call this set of labels a "series identifier".

An instant vector of type `a` is a set of `instant`s of type `a` where every two elements of the set have distinct:
— Set of labels

A timeseries of type `a` is:
— A set of labels
— a set of pairs of:
  — A timestamp
  — An element of `a`

A timeseries vector of type `a` is a set of timeseries where every two elements of the set have distinct:
— Set of labels

