Function compact 

pub fn compact(changes: &[Change]) -> Vec<Change>

Coalesce redundant changes while preserving the apply result. Only safe reductions are performed:

• repeated writes to the same node field (SetText/SetMarks, or SetAttr/RemoveAttr / SetExtra/RemoveExtra on the same key) collapse to the last one (each write fully determines that field’s final value);
• an Insert immediately followed by a Remove of that same just-inserted child cancels out.

Structural ops (Insert/Remove/Move/Replace) act as barriers: because [apply] resolves indices against the live tree, field ops are never coalesced across them. compact(c) is never longer than c.

Precondition: this preserves the apply result for change lists that apply cleanly to their intended base (as produced by diff / Transform). For a deliberately invalid list it can differ — e.g. an out-of-bounds Insert immediately followed by its matching Remove cancels here, whereas applying the original would error; validity can’t be checked from a change list alone.