setup for call-by-need
added ex. 4.40
added TRAMPOLINE (=LETREC-CONT) before
implemented trampoline
added ex. 5.17
wrapping up trampoline
adapted example of to show exceptions :-)
added ex. 5.35
added ex. 5.38 (skipped 5.36, 5.37)
skipping most of threads for now
added ex. 7.6
added ex. 7.7 (example to play with)
added  ex. 8.1
added ex. 8.2
prefer (list 3) over '(3)
added ex. 9.6
added ex. 9.5
using otherwise unused assq-method-env func. ;-)
added some inference verifications
removed redundant let ;-)