EXPRESS_ — EXPRESS MM Issue: Definition of Constant

In clause 9.6.1 Constants are said to be Instances in the definition, but Constant has only one supertype: CommonElement; Constant is not modeled as a subtype of Instance. The model only shows a Constant to be related to an Instance by :actual-value. One of the model and the definition must be wrong.

The model is correct. Constant is not a subtype of Instance. The definition says that a Constant "denotes" an Instance. It behaves more like an initialized variable in that it has a lifetime and represents a given instance during its lifetime. Only Constants that are SchemaElements behave like Instances. The text will be revised to clarify this.
The appearance of Constant in the Instance Package is misleading. Constant should not be in the Instance Package, precisely because it is not an Instance. Because Constants, especially Constants whose value-expressions are Literals, can be used in formulating datatypes, Constant could be moved to the Core package or to the Expressions package.
Also, the :actual-value relationship should be derived from the value-expression