There are several classes to represent price indexes.
All indexes inherit from the piar_index virtual class.
Period-over-period indexes that can be chained over time inherit from
chainable_piar_index.
Fixed-base indexes inherit from direct_piar_index.
The piar_index object is a list-S3 class with the following
components:
A list with an entry for each period in time that gives
a vector of index values for each level in levels.
A list with an entry for each period in time, which
itself contains a list with an entry for each level in levels with
a named vector that gives the percent-change contribution for each price
relative.
A character vector giving the levels of the index.
A character vector giving the time periods for the index.
The chainable_piar_index and direct_piar_index subclasses have
the same structure as the piar_index class, but differ in the methods
used to manipulate the indexes.