Rewrites Tutorial Exercise 5

If x is the sum a + b, then `nterms(x)' must be `nterms(a)' plus `nterms(b)'. If x is not a sum, then `nterms(x)' = 1.

[ nterms(a + b) := nterms(a) + nterms(b),
  nterms(x)     := 1 ]

Here we have taken advantage of the fact that earlier rules always match before later rules; `nterms(x)' will only be tried if we already know that `x' is not a sum.