operator in <tt>(%i9)</tt> means "do not evaluate".  Without
it, Maxima would have obtained 0:

<pre>
(%i10)&nbsp;diff&nbsp;(y,&nbsp;x);
(%o10)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;0

</pre>
Using the quote operator we can write differential equations:

<pre>
(%i11)&nbsp;'diff&nbsp;(y,&nbsp;x,&nbsp;2)&nbsp;+&nbsp;'diff&nbsp;(y,&nbsp;x)&nbsp;+&nbsp;y;
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp