[R] Superscripts in strip labels of lattice plot

[Judith Flores]

Hello,

   I need to add superscripts of the strip labels of a xyplot. 

For example: 


xyplot(a~b|c)

   Where c has 3 factor levels: A^2, A^3 and A^4. How can I introduce 
expressions in the strip labels?

Thank you very much!

Judith