There are many definitions of type (and class and related concepts).  Many
authors define the terms as applied by their particular approach or language,
however we shall proceed in the face of this diversity.

  References
    [Blair 89]          Some Typing Topics.
    [Booch 91]          Small Section on Typing.
    [Cardelli 85]       Discussion on Object-Oriented Typing.
    [Gunter 94]         Theoretical Aspects of Object-Oriented Programming.
    [Kim 89, ch1]       Discussion on Some Research Topics.