nt to cos
("phase modulation", <code>cos(angle + change)</code>), or add it to the current phase, then take cos of that 
("frequency modulation", <code>cos(angle += change)</code>), so
our formula can viewed either way.  Since the angle is being incremented by the carrier frequency in
either case, the difference is between:
</p>
<pre class="indented">
PM: cos((angle += incr) + change)
FM: cos(angle += (incr + change))
</pre>
<p>
To make the difference clear, textbooks
put in an integral when they mean 
freq