<= =>
Elementos Não Avaliados
		      
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 : _