[Maxima] Defining a derivative

[Nicolas FRANCOIS]

Hi.

I have a problem with Maxima : I define a function r, and then I want to
nam its derivative. But the code :

r1(t):=diff(r(t),t)

doesn't work if I use r1 with for example 

r1(2)

r1(t=2)
 -- an error.  Quitting.  To debug this try debugmode(true);

How can I define r1 to prevent this ? For example MuPAD had a "D"
operator for this. But I didn't find the equivalent in Maxima.

Thanks for any help.

\bye

-- 

                   Nicolas FRANCOIS
            http://nicolas.francois.free.fr
 A TRUE Klingon programmer does NOT comment his code