Exercise 3.55.  Define a procedure partial-sums  that takes as argument 
a stream S and returns the stream whose elements are S0, S0 + S1, S0 + 
S1 + S2, .... For example, (partial-sums integers) should be the stream 
1, 3, 6, 10, 15, ....

————————————————————————————————————————————————————————————————————————

(define (partial-sums s)
  (define stream (cons-stream 0 (add-streams s stream)))
  (stream-cdr stream))