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                <mn>-1</mn>
              </math>, are accepted.</p></dd><dt><span class="term"><em class="parameter"><code>equation</code></em></span></dt><dd><p>Specifies the address of an array of four fixed-point or
          floating-point values. These are the coefficients of a plane
          equation in object coordinates: <span class="emphasis"><em>p1</em></span>,
          <span class="emphasis"><em>p2</em></span>, <span class="emphasis"><em>p3</em></span>, and
          <span class