ex 4.41 - done
ex 4.41 - first impl
ex 4.41 - done
ex 4.41 - complete comment
ex 4.42 - impl
ex 4.42 - answer and done
ex 4.43 - a draft about relations
ex 4.43 - not working.
ex 4.43 - working impl, need speed up
ex 4.43 - optimize
ex 4.43 - q1 done
ex 4.43 - done
ex 4.44 - 8 queen impl
ex 4.44 - impl pretty print, done