gher-order differences, such as the third difference, must be expressed explicitly, e.g., <code>C_s - C_s[-3]</code>.</li>
<li>
<strong>Lagged differences</strong>: Combine the above two operations, e.g., <code>d(C_s[-1])</code>.</li>
</ul>
</li>
<li>Each variable being defined can appear on the left-hand side of a single equation. It is not allowed to have multiple different equations for the same variable (exact duplicates are flagged with an appropriate message).</li>
</ol>
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