Sorry, I've found than 'inject' has form without argument and it does the job.

module Enumerable
 def sum
   inject {|f,x| f+x}
 end
end

But i am still interested in:
1) Is it a good idea to add method ''first' to  Enumerable?
2) Please, show me the _right_ code for Enumerator#skip_first(n=1) (ruby1.9)


And one more question offtopic:

I see my messages duplicated in list. I use gmail.com
What should I do to stop duplicating my messages?