Plotting the heaviside function



Hello

I have defined the function :
f(x):=if x<0 then 0 else 1
but maxima can't plot this function :

plot2d(f(x),[x,-2,2]);
returns :
MACSYMA was unable to evaluate the predicate:
x < 0
#0: f(x=x)
 -- an error.  Quitting.  To debug this try DEBUGMODE(TRUE);)

How can i do ?
Best regards
-- 
****************************
Michel Gosse 
michel.gosse@ac-poitiers.fr
michel.gosse@freesbee.fr
****************************