Daniel-Nikpayuk_SICP-Solutions/Chapter 3/3.5/exercise-3.57/exercise.lsp
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;; Copyright 2015 Daniel Nikpayuk
;;
;; This file is part of SICP Solutions.
;;
;; SICP Solutions is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License
;; as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
;;
;; SICP Solutions is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty
;; of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
;;
;; You should have received a copy of the GNU General Public License along with SICP Solutions. If not, see
;; <http://www.gnu.org/licenses/>.
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(define fibs
  (cons-stream 0
               (cons-stream 1
                            (add-streams (stream-cdr fibs)
                                         fibs))))

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;; So with the third element you add streams, and you calculate the first and second elements of fibs to do so.
;; With the nth element you add streams, and you calculate the n-2th and n-1th elements of fibs to do so.

;; So there's a delayed promise to calculate the second element of add-streams which is the fourth element of fibs.
;; But to get this 4th element of fibs you need to access the second and third elements of fibs, which are on delays
;; themselves, and so without call-by-need the number of calculations grows exponentially.

