Math preview failure in UserGuide

[Jean-Marc Lasgouttes]

When I load UserGuide with previews enabled, I see a lot of errors on 
console:

Warning: dvipng failed to generate images from lyxpreviewjVAhXp.dvi... 
fallback to legacy method
Warning: epstopdf failed on page 1, file lyxpreviewjVAhXp.001
Warning: epstopdf failed on page 2, file lyxpreviewjVAhXp.002
Warning: epstopdf failed on page 3, file lyxpreviewjVAhXp.003
Warning: epstopdf failed on page 4, file lyxpreviewjVAhXp.004
Warning: epstopdf failed on page 5, file lyxpreviewjVAhXp.005
Warning: epstopdf failed on page 6, file lyxpreviewjVAhXp.006
Warning: epstopdf failed on page 7, file lyxpreviewjVAhXp.007
Warning: epstopdf failed on page 8, file lyxpreviewjVAhXp.008
Warning: epstopdf failed on page 9, file lyxpreviewjVAhXp.009
Warning: epstopdf failed on page 10, file lyxpreviewjVAhXp.010
Warning: epstopdf failed on page 11, file lyxpreviewjVAhXp.011
Warning: epstopdf failed on page 12, file lyxpreviewjVAhXp.012
Warning: epstopdf failed on page 13, file lyxpreviewjVAhXp.013

What heppened? Well, if I understand correctly, the preview cannot be 
generated because of the too clever trick in section 6.9 Branches, where 
branches are used to declare macros. Of course, the preview mechanism 
does not see the ERT that declares of macros in branches and compilation 
fails.

Two things:

1/ shall I change the example to something more reasonable that does not 
upset math previews (especially since a proper trick with math macros is 
already proposed in Math Manual)?

2/ can we avoid, when we see that dvipng failed (because there is no dvi 
file?), to run these nonsensical epstopdf commands?

JMarc