Stacking things in LaTeX/LyX

Sometimes you need to put one thing on top of another. A good example is when the indices over which to sum are a complicated intersection of sets. In order to have them appear nicely below the summation consider using \substack as opposed to \atop. The Internet is rife with documentation for these commands. I will just mention the use of \substack in LyX:

In a math mode box you can type \substack. This will turn into the substack mode box. Here you can type your first line. To get the next line just hit Ctrl+Return. This will send you into the next line where you can type your line. I guess you can continue in this fashion to some point limited by some cryptic limit that has been hard-coded into the innards of LyX. ;p