    len [1,2,3,4,5]
==> 1 + len [2,3,4,5]
==> 1 + (1 + len [3,4,5])
==> 1 + (1 + (1 + len [4,5]))
==> 1 + (1 + (1 + (1 + len [5])))
==> 1 + (1 + (1 + (1 + (1 + len []))))
==> 1 + (1 + (1 + (1 + (1 + 0))))
==> 1 + (1 + (1 + (1 + 1)))
==> 1 + (1 + (1 + 2))
==> 1 + (1 + 3)
==> 1 + 4 
==> 5