# GTR009 — Upgrade24_0: hoist identical element filters (24.0 → 24.1)

**Contract:** behaviour-preserving restructure (equivalence by Galaxy's filter
semantics).

## The delta
24.1 forbids `<filter>` inside a collection's child `<data>` (only
`actions`/`change_format` remain); top-level output `<data>` filters stay legal.

## The transform
When **every** child `<data>` of a collection carries the **same** `<filter>`
condition, hoist one copy to the `<collection>` and drop the per-element ones.

## Proof
A filter is a Python condition deciding whether the output appears. Identical
conditions on *all* elements ⇔ one condition on the whole collection: in both
forms the collection's elements exist exactly when the condition holds. Galaxy
evaluates filter **text** — `eval(filter.text.strip())` — which also settles
the 2026-06-04 adversarial claim: a post-comment tail inside `<filter>` is dead
at runtime, so the hoist dropping it changes nothing (refutation overreach,
re-verified, PR #115).

## Scope boundary (bails, reported by the discovery sweep)
- differing child filters (genuine per-element selection);
- partially-filtered collections (hoisting would newly filter the rest);
- a collection already carrying its own filter (hoisting would AND a second
  condition).

## History
Codemod §14; the refutation-overreach record lives in the ledger.