finish exercise 4.8 ~ 4.9
finish exercise 4.11
finish exercise 4.12~4.13
start project 5, changed some codes and run it correctly
in project5, add some tests for' m-eval'
in project5, finish computer exercise 1.
in file environment.scm, add some primitive procedures.
in file mycode.scm, test these primitive procedures.
add tests for exercise 4.1
add tests for exercise 4.2
add tests for exercise 4.3
add tests for special form of 'and' in exercise 4.4
todo: add tests for special form of 'or'
add other tests for exercise 4.4
fix exercise 4.5, add tests.
fix exercise 4.6, add tests.
add tests for exercise 4.7~4.8
add tests for exercise 4.10, but find another bug:
the while-expression nested in let-expression cannot work.
todo : fix the bug
finish the test of exercise 4.9, fix the bug.
note: the previous commit message is wrong, i'ts for exercise 4.9 not 4.10
finish the tests for exercise 4.11 in file 'e4.11-.scm'
finish exercise 4.12 and test it.
finish exercercise 4.13 and test it.
finish exercise 4.14