# GTR001 — Canonical 4-space indentation (fmt)

**Contract:** behaviour-preserving.
## Claim
Rewriting whitespace-only `.text`/`.tail` to canonical indentation never
changes runtime or rendered behaviour.

## Proof
- `<tool>` and other **element-only content models** (XSD): XML 1.0 declares
  inter-element whitespace non-significant in element content — rewriting it is
  invisible to any conformant consumer.
- The rule **skips the whole subtree** of (a) any element holding **mixed
  content** (a ws-only tail there is a rendered word separator — the XML 1.0
  clause covers element content only) and (b) any **text-bearing element with
  children** per the schema-derived payload guard (`payload.py` over tier-1
  `schema_content.text_bearing_tags()`): even an all-whitespace text/tail
  between `<expand>` children is spliced into script/template/RST, where a
  space→newline rewrite changes meaning (in shell, a newline *separates
  commands*).
- `safe_set_text`/`safe_set_tail` write only when the existing value is
  None/whitespace.

## Scope boundary
Whitespace inside text-bearing subtrees is never touched (fmt §D19/§D20). The
guard set is derived from all 28 vendored XSDs, with two proof-carried
exceptions documented in [GTR004](GTR004.md).

## History
Refuted (mixed-content drift) → guarded, PR #158; payload-guard derivation fmt
§D20. Audit trail: `../behavior_preservation.md`.