question about behavior of 'hipow'

Hi,

I'm confused about the behavior of 'hipow' when the first argument contains
derivatives. For example consider

(%i1) depends(x,t);
(%o1)                               [x(t)]
(%i2) hipow(diff(x,t)^2,x);
(%o2)                                  1
(%i3) hipow(diff(x,t)^2,diff(x,t));
(%o3)                                  2
(%i4) hipow(diff(x,t)^2,diff(x,t,2));
(%o4)                                  0

Output lines %o3 and %o4 make sense to me but I don't understand why
hipow(diff(x,t)^2,x) should evaluate to 1, output line %o2. Is there
something I don't understand about hipow? I'm using Maxima 5.20.1.

Many thanks for your help.

-- 
Todd