Fails xelatex:
```
Runaway argument?
 \begin {split}a \frac {df(x,y)}{dx} + b \frac {df(x,y)}{y) + c f(x,y\ETC.
! Paragraph ended before \gather was complete.
<to be read again> 
                   \par 
l.104242 
         
? 
```
Notice the mistyped closing brace in `\frac{df(x,y)}{y)`