tifier">x2</span> <span class="special">*</span> <span class="identifier">fx1</span> <span class="special">+</span> <span class="identifier">x2</span> <span class="special">*</span> <span class="identifier">x1</span> <span class="special">*</span> <span class="identifier">x1</span> <span class="special">-</span> <span class="identifier">x1</span> <span class="special">*</span> <span class="identifier">x2</span> <span class="special">*</span> <span class="identifier">x2</span><span class="special">)</span> <