Roadmap

go-ruby-rexml/rexml is grown test-first, each capability differential-tested against MRI rather than built in isolation. Ruby's REXML — the deterministic, interpreter-independent slice extracted from rbgo's internals — is complete.

Stage	What	Status
Parse	ParseDocument building the full node tree, preserving raw text / entities / XML declaration / DOCTYPE, returning a *ParseError with byte offset on malformed input.	Done
DOM	Element accessors and mutators (Add*, Elements, EachElement, Text, ElementAt, QName / NamespaceURI, Root), with an ordered Attributes map.	Done
Serialise	Compact ToString, Write(WriteOptions), and the Pretty formatter — exact attribute quoting / ordering, entity escaping, CDATA / comment / PI round-trip, self-closing empties.	Done
XPath subset	XPathFirst / XPathEach / XPathMatch over the widely-used subset; axes / functions outside the documented boundary are out of scope.	Done
Node model	The concrete Document / Element / Attribute / Text / Comment / CData / Instruction / DocType / XMLDecl types a host binds, with raw round-trip fidelity.	Done
Differential oracle & coverage	A wide XML corpus re-serialised here and by the system ruby, compared byte-for-byte; 100% coverage, gofmt + go vet clean, green across all six 64-bit Go arches.	Done

Documented out-of-scope boundaries

These are deliberate, recorded so the module's surface is unambiguous:

• XPath subset boundary. Only the widely-used subset is implemented: axes beyond child / descendant (no ancestor, following-sibling, parent, ..), functions beyond text() (no count(), position(), last(), name(), contains(), …), arithmetic, and compound boolean predicates (and / or) are out of scope; an unrecognised predicate leaves the node set unchanged.
• No interpreter. The library implements the deterministic parse / serialise / XPath algorithm; it never runs arbitrary Ruby. Anything needing a live binding is the consumer's job — that is why rbgo binds this module rather than the reverse.
• Reference is reference Ruby (MRI). Byte-for-byte conformance targets MRI's rexml 3.4.4, pinned by the differential oracle.
• Standalone & reusable. The module has no dependency on the Ruby runtime; the dependency runs the other way.

See Usage & API for the surface and Why pure Go for the deterministic/interpreter split.