saving functions



Hi - 

This is probably a very simple question, but one that I don't know the 
answer to nonetheless.

I have used Maxima several times to write small functions, such as a 
parallel resistance function

P(Z1, Z2):=ratsimp(1/(1/Z1+1/Z2))

It's not hard to write something like this, but I would really prefer 
to not have to every time.  Again, I know this is a simple question, 
but how do you save and load user-defined functions such as the one 
above?  Thanks.

-Ron