[MonetDB-users] how to write function like oracle nvl()?

Hi, I need some help on how to write some special functions that monetdb currently doesn't have. E.g., I want to use the oralce funtion nvl(), which is described as follows: http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/functions105.... Then, how can I write a function that has the some ability as the oralce's? By the way, now I use MonetDB-Feb2010.