starting chapter 2.2

represent sequence using pair
operations on list

ref, length, append
solve exercise 2.17
solve exercise 2.18: reverse a list
solve exercise 2.19: change-counting problem
solve exercise 2.20 function which take arbitrary numbers of arguments
map over sequence
solve exercise 2.21 map implementation
solve exercise 2.22
solve exercise 2.23 for-each
start hierarchical structures,count leaves of a tree
solve exercise 2.25
solve exercise 2.26
solve exercise 2.27 deep reverse
solve exercise 2.28 collect leaves of a tree
solve exercise 2.29 binary mobile
map over trees
solve exercise 2.30 square-tree
solve exercise 2.32 subsets of a list
conventional interfaces: map filter accumulate