I should use 'AMS multiline'?
sure
Why use multline, rather than what you get when you just press C-Enter? (Or perhaps the question should be why don't we get multline?)
how should LyX know what kind of equation the user wants, when he inserts C-Enter?? Every math environment has its sense and in this case, one equation over several lines, its multline. Compare the horizontal alignment with the other environments.
And in the last example you get different sizes of the equations, also no error, but looks ugly.
Thank's for the example file, I uploaded it as well.
I noticed that you used \vphantom{} to fix the size of the brackets... that's sort of like a hack to me. Is this the only way?
what do you think, what's the sense of vphantom??? This is no hack, it is the LaTeX way of getting right parenthesis. Another way is to use static parentheses, line \bigg( a.s.o Look at my math documentation.
Herbert
