plan to use racket to actually impl sth.
ex 3.46 - analyze
ex 3.46 - done
3.4.2 - actually we have the impl of `clear!`
ex 3.47.a - done
ex 3.47.b - done
not needed probably
start working on an extra task
bank account exchange - basic functions
threads working
comment + some tests
test 2
ex 3.48 - stark working, same task.
ex 3.48 - done
bank account - the last test & done
ex 3.49 - done
3.4 - done
3.5 stream
3.5.1 - started
3.5.1 - streams are delayed lists