Should be able to add a pipeline unit at a given place

Currently, since #168 , one can add pipeline units at the end of a pipeline.
One should also be able to add them at any place.

Start by inspecting the pipeline-unit addition introduced for #168 and trace how a unit's position is represented. Clarify the expected behavior for insertion at an arbitrary place, then verify that the pipeline accepts positions other than the end while preserving existing ordering.

Written by the indexing model from the issue text.