This is "OSF Version 0" defining a simple OSF constraint language. It follows the main lines of the basic specification.

This language consists of a bare-bone syntax and operational semantics for order-sorted feature (OSF) terms - also known as ψ-terms - and basic operations for them. It first focuses on defining a multiple-inheritance concept hierarchy of sort symbols. It implements the machinary for compiling these sorts into binary codes to optimize lattice operations on them. This will be used in later versions when implementing ψ-terms unification and generalisation.

Essentially, this simple language is meant as an initial prototype for declaring a sort lattice, implementing binary encoding of sort ordering using bit vectors, evaluation of sort expressions, and reading and pretty-printing ψ-terms. It also offers interactive pragmas for exploring a sort hierarchy. This initial version does not support OSF term unification or generalisation, term disjunction nor negation yet (other than on sort symbols), no residuation, no OSF theories, and no monoidal aggregation. It does not provide rules either (whether logic programming or functional rewriting). Also, the ψ-terms are kept in their original labelled-graph form (i.e., they are not dissolved into basic OSF constraints). All this will come gradually in later versions extending this basic one.