Rules for Identifiers

An optional relationship membership may not participate in an identifier.
A transferable relationship membership may not participate in an identifier.
A relationship membership that is involved in a mutually exclusive set may not participate in an identifier.
The initial value of an attribute that participates in an identifier may not be changed.
An optional attribute or relationship may not participate in an identifier.
A derived attribute may not participate in an identifier.
A BLOB attribute may not participate in an identifier.