>opcode</code>
                </em>
              </span>
            </dt>
            <dd>
              <p>
                    Specifies a symbolic constant that selects a logical operation.
                    The following symbols are accepted:
                    <code class="constant">GL_CLEAR</code>,
                    <code class="constant">GL_SET</code>,
                    <code class="constant">GL_COPY</code>,
                    <code class="constant">GL_COPY_INVERTED</code>,
               