Word 2007 Equations: How to stack symbols
Solution 1:
You can use \above and \below:
a=\above("foo") b
becomes
a=┴"foo" b
which becomes
A more or less complete description of the formula language in Word is given in Unicode Technical Note #28, although the final implementation differs somewhat in some areas. Unfortunately this is the only real documentation that's there (I asked the author of the math typesetting stuff at MS :-)).