Newbie - integrate piecwise function



Hi, I'm a new Maxima user (5.9.0 on Debian GNU/Linux).

After reading through the docs, I'm still puzzled by the following:

if I define f(x) as follows 

f(x) := IF x < 0 THEN 0 ELSE 1;

then 

integrate(f(x), x ,1,2);

gives the error 

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


Is there a "proper" way to integrate piecewise functions in Maxima?


Pieter