/ provides stackrel for placing a
superscript above a binary relation.
In the amsmath package there are somewhat more general commands,
overset and underset, that can be used to place one symbol
above or below another symbol, whether it's a relation or something
else. The input |*X| will place a superscript-size 
above
the X: 
; underset is the analog for adding a
symbol underneath.
See also the description of sideset in sideset.