Lack of an explicit map between the nonterminal symbols of the EXPRESS grammar and the concepts in the EXPRESS metamodel is a significant impediment to implementation of the EXPRESS Metamodel. This has really complicated the business of building an EXPRESS compiler that can populate STEP schemas or rewrite EXPRESS text from models of STEP schemas. The mapping defines an implementation's interpretation of the EXPRESS syntax, which is well-defined in ISO 10303-11:2004. The mapping needs to be 2-way, idempotent.

The comments about the mapping that appear in the body of the specification are inadequate. There are 342 production rules to be mapped to about 265 metaclasses. Many of these mappings are quite obscure, indirect, difficult to conjure. To assure interoperability of implementations, implementations need to use the same mapping. The mapping should be part of the specification.

You might get more uptake if you were to include such a map as a normative Annex and XML file.

The OCL in the specification regularly uses the -> operator incorrectly. The . operator applies to an individual and -> applies to a collection.

The syntax of references to collections should also be checked.