extending of adding method?

[Gerardo Herzig]

Hi all. I have a question about design. The general question would be: 
Where is the line beteween extending a class and adding new methods to 
that class?

And the particular case im involved is:

I have a class (say USERCLASS) who implements a user interface. One of 
his methods is called `login'. Later on i got the order of implementing 
a `single login', which off course means a single user cant login if a 
session is already started for that user. Extending comes to my mind at 
first (it works good, actually). But now im thinikng if there some 
problem in adding a method to the USERCLASS class named single_login(), 
who will take care of applying the rules of single sign on.

Which one you guys will pick up? USERCLASS().single_login() or 
SINGLELOGINUSERCLASS().login()


Thanks for your oppinions!
Gerardo