Where is IFNULL?

Hi,

the user manual describes the function IFNULL on page 38, but when I
try to use it I get the following error:

  users=> select ifnull (NULL, 'nix');
  ERROR:  No such function 'ifnull' with the specified attributes

What do I have to do to get the function?

Or does anybody know a simple way to concatenate strings, which can be
NULL?

Sascha