The hyperspec tells me that the array dimensions should be fixnums>=0.
I also checked the function array-dimension, this time the hyperspec says
it returns an integer.  Is this an oversight or am I missing something?  
I got curious about that when I discovered in code like

(dotimes (j (array-dimension foo 0))
    (do-something))    

j needed a fixnum declaration for the implicit increment operation to
be open coded.  I thought this was due to insufficient type-inference
on ACL, but apparently the hyperspec does not sanction what I was
after.  (I don't imagine the standard itself is different).  

???