``Successful application of a reuse technique to a software engineering technology is inexorably tied to raising the level of abstraction for that technology.''

Every software abstraction has two levels:

  1. Specification

  2. Realization
If there are multiple levels, the realization of one level may be the specification of the next lower level.

An abstraction has several parts:

  1. hidden part: details of the realization that are not visible in the specification

  2. variable part: the part of the specification that can be varied

  3. fixed part: the part of the specification that is fixed

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