pattern matching with keyword function

Hi,

I have "stumbled" on this simple function

let rec flatten = function
     [] -> []
   | l::r -> l @ flatten r

At first glance I asked myself why they don't use the last remaining 
element of the list
then I see that the line
     [] -> []

doesn't have any "|" at the beginning.

Could you explain we how does that works?
And maybe could you give me one or more links for informations/examples 
on the patterns matching
  with the function keywords.

Thanks

cedlemo