displa for sub and superscripted functions



> ((mexpt) (($P simp array) ,(nth 1 form)) 
>          (,(nth 2 form) ,(nth 3 form)))

The only thing that should be in the CAR of a Maxima expression is the
operator part, e.g. (mplus).  In this case, you can use MPROGN, which
displays as (a,b,c):

> ((mexpt) (($P simp array) ,(nth 1 form)) 
>          ((mprogn) ,(nth 2 form) ,(nth 3 form)))