Expand description
A plugin’s moves, carried out on a copy of the region trees as the plugin makes them. The copy lets a plugin name what its own moves create (the piece a cut leaves, the fold a join adds) with the ids the applier will give them, since both hand out ids the same way.
The helpers here are the moves the bundled plugins make together: collapsing a region under a label, linking regions, cutting lines out of a leaf, and grouping siblings under a labelled fold.