cases environment does not exist in the default document style.

[Bo Peng]

I start lyx, create a new document, insert math->cases environment, view 
dvi. I get two ERRORS!

This is because the default document style article does not have 
amsmath. The solution is either adding 

  \usepackage{amsmath}

in the preamble or using another document style like article(AMS).

This is not a big prolem but it would be better to avoid this kind of 
problem by adding amsmath to the default style.

Thanks.

-- 
Bo Peng