Add new columns?

Apparently people often want to add new columns that are results from the old columns.

Do we want to support adding another column onto an existing castra? We would assume that the new column has the exact same partition structure and number of elements per partition.

The issue names no files, tests, or entry points. Start by locating the existing castra creation and column-writing APIs, then determine whether an existing castra can accept a derived column while preserving its partition structure and elements per partition. Done means the supported behavior and its constraints are defined and verified.

Written by the indexing model from the issue text.