><span class="va">x</span><span class="op">)</span> <span class="op">{</span>
  <span class="va">assert_op</span> <span class="op">&lt;-</span> <span class="fu"><a href="../reference/tf_assert.html">tf_assert</a></span><span class="op">(</span><span class="va">x</span> <span class="op">&gt;</span> <span class="fl">0</span>, <span class="st">"x is positive"</span><span class="op">)</span>
  <span class="fu"><a href="https://rdrr.io/r/base/with.html">with</a></span><span class="op">(</span><span class="va">tf