Optional Readonlychecknode -> WriteReport; simulates a write without producing output.
ReadonlynameRegistered format name identifier (e.g. "json", "yaml", "oml").
Readonlyreadtext -> node. opts is format-specific (e.g. XML's { report } for
read-time codec diagnostics, issue #123/D-3); most formats ignore it.
Readonlywrite(node, opts?) -> text
A registered format plugin: a name plus read/write/check callables.