weight: bold;">Contraction</span>: Any
(closed)
theory can be contracted or reduced to a smaller, simpler closed theory
by deleting one or more axioms.</p>
  </li>
  <li>
    <p><span style="font-weight: bold;">Expansion</span>: Any
(closed)
theory can be expanded by adding one or more axioms. </p>
  </li>
  <li>
    <p><span style="font-weight: bold;">Revision</span>: A revision
step
is composite &#8211; it uses a contraction step to discard irrelevant
details, followed by an expansion step to added new ax