object identify:
An object retains its identity even if some or all of the values of variables or definitions of methods change over time.
Object identity is typically implemented via a unique, system-generated OID. The value of the OID is not visible to the external user, but is used internally by the system to identify each object uniquely and to create and manage inter-object references.

**object structure**:
http://www.cs.sfu.ca/CourseCentral/354/zaiane/material/notes/Chapter8/node4.html

Type constructor:
atom, tuple, set, list, array, bag


source:
http://people.cs.pitt.edu/~chang/156/19oodb.html