class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal">x</span><span class="mopen">(</span><span class="mord mathnormal">t</span><span class="mclose">)</span><span class="enclosing math-punctuation"><span class="mord text"><span class="mord">,</span></span></span></span></span></span> we can think of the identity <span class="katex"><span class="katex-mathml"><math><semantics><mrow><mi>x</mi></mrow><annotation encoding="application/x-tex">x</annotati