ex 3.15 - done
ex 3.16 - working
ex 3.16 - done
ex 3.17 - done
ex 3.18 - impl
ex 3.18 - done
utils - new proc: take
ex 3.18 - remove unused
ex 3.19 - done
3.3.1 - mutation is just assignment
ex 3.20 - get started
ex 3.20 - working
ex 3.20 - done
3.3.2 - queue abstraction
3.3.2 - code copied, want some test...
3.3.2 - queue example
3.3.2 - few comment, + readability
ex 3.21 - startup ...
seperate queue impl as lib
ex 3.21 - reproduce problem