ex3 = printO (length ((observe "lista" :: Observing [()])
[error "Oops!" | _<-[0..9::Int]] ) )
-- lista
_ : _ : _ : _ : _ : _ : _ : _ : _ : _ : []
ex4 = runO a
where a = print(xs !! 2 + xs !! 4)
xs = observe "lista" [0..9::Int]
-- lista
_ : _ : 2 : _ : 4 : _