Tài liệu Research Issues in Systems Analysis and Design, Databases and Software Development phần 6

The main idea behind ORC, as present in its early ancestor RIDL (Meersman,
1982), is a functional, variableless description of domain-specific properties
(and queries). RIDL does contain a linguistic reference mechanism (the demonstrative
THAT). In ORC, variables have been introduced to handle more
subtle referential relations that cannot be handled by demonstratives. Variables
are special names that are instantiated once they are evaluated in a context
that generates values for these variables. The concept of environment is used
to administrate the value of variables. In environment E, the variable v will
evaluate to E(v). Some examples of the use of variables follow