? newsymbol{"!le"}{"≰" "\not\leq"} a !le b
<math title="newsymbol{&quot;!le&quot;}{&quot;&amp;#x2270;&quot; &quot;\not\leq&quot;} a !le b">
  <mstyle mathcolor="red" displaystyle="true" fontfamily="serif">
    <mi>a</mi>
    <mo>&#x2270;</mo>
    <mi>b</mi>
  </mstyle>
</math>
$$a \not\leq b$$

? newsymbol{"!le"}{"&#x2270;"} a !le b
<math title="newsymbol{&quot;!le&quot;}{&quot;&amp;#x2270;&quot;} a !le b">
  <mstyle mathcolor="red" displaystyle="true" fontfamily="serif">
    <mi>a</mi>
    <mo>&#x2270;</mo>
    <mi>b</mi>
  </mstyle>
</math>

? newsymbol{"(R)"}{"&#x00AE;" "\textregistered"} (R)
<math title="newsymbol{&quot;(R)&quot;}{&quot;&amp;#x00AE;&quot; &quot;\textregistered&quot;} (R)">
  <mstyle mathcolor="red" displaystyle="true" fontfamily="serif">
    <mo>&#x00AE;</mo>
  </mstyle>
</math>
$$\textregistered$$

? newcommand "DDX" "{:d/dx:}" DDXf(x) = f'(x)
<math title="newcommand &quot;DDX&quot; &quot;{:d/dx:}&quot; DDXf(x) = f'(x)">
  <mstyle mathcolor="red" displaystyle="true" fontfamily="serif">
    <mrow>
      <mfrac>
        <mi>d</mi>
        <mrow>
          <mi>d</mi>
          <mi>x</mi>
        </mrow>
      </mfrac>
    </mrow>
    <mrow>
      <mi>f</mi>
      <mrow>
        <mo>(</mo>
        <mi>x</mi>
        <mo>)</mo>
      </mrow>
    </mrow>
    <mo>=</mo>
    <mrow>
      <mi>f</mi>
      <mo>'</mo>
    </mrow>
    <mrow>
      <mo>(</mo>
      <mi>x</mi>
      <mo>)</mo>
    </mrow>
  </mstyle>
</math>
$$\frac{d}{d x} f ( x ) = f ' ( x )$$

? newcommand "FUNKY" ""

? newcommand "NAME" bad
<math title="newcommand &quot;NAME&quot; bad">
  <mstyle mathcolor="red" displaystyle="true" fontfamily="serif">
    <mrow>
      <mo>newcommand</mo>
      <mrow>
        <mtext>NAME</mtext>
      </mrow>
      <mi>b</mi>
    </mrow>
    <mi>a</mi>
    <mi>d</mi>
  </mstyle>
</math>
$$\mbox{newcommand} \mbox{NAME} b a d$$