LaTeX.org

Nope. Even this doesn't work. \sideset is ONLY designed to be used with large operators such as \sum and not on normal symbols.

Basically... LaTeX has a HUGE glaring absence of a commonly needed math typesetting operation. Nice job. All you Chemistry ppl can go take a leap too; why would you ever ...

WRONG! just plain WRONG! please try to think a little about what the author might be trying to do other than just $^i_jX$ .

$^AP = ^BP^A_BT$ (a common linear algebra problem in
robotic kinematics for instance) comes out COMPLETELY wrong. TeX WILL associate the second ^A with the P just before it ...