A theoretical example

• The main data structure, a gb_tree with:
  [{0, %{user: :a}}, {1, %{user: b}}, {2, %{user: z}}, {3, %{user: a}}
• The index, a Map of lists with:
  %{a: [0, 3], b: [1], c: [2]

You then iterate over the gb_tree until you find something you want to delete/process; then, to find out which other entries you need to delete, you consult the index rather than iterating over all the entries.

Then, once you’ve finished processing that particular group of objects, you can continue iterating where you left of (gb_tree does provide a function for this.)

All that it’s left is to write the bookkeeping code necessarily to ensure the two structures remain consistent with each other for every write.