Round-Trippable RDF 1.2 Interoperability

RDF 1.2 introduces triple terms, enabling an RDF triple to occur in the object position of another triple. This extension yields two conformance profiles: RDF 1.2 Full, allowing triple terms, and RDF 1.2 Basic, excluding them. Consequently, interoperability between Basic and Full implementations becomes non-trivial, as Basic processors cannot directly consume graphs containing triple terms. In response, the W3C Group Note Draft RDF 1.2 Interoperability, published by the RDF & SPARQL Working Group, describes a non-normative pair of transformations: basic encoding (Full -> Basic) and a corresponding decoding (Basic -> Full). This paper provides a mathematically precise account of that proposal, grounded in the RDF 1.2 abstract syntax and RDF 1.2 model-theoretic semantics. We formalize the syntax of triple terms, define recursive "appearance" of terms, specify encoding/decoding as functions on graphs and datasets, establish invertibility and idempotence results under explicit admissibility conditions, and briefly clarify their semantic status under standard RDF semantics.