2.1.1 - done
ex 2.1 - done
correct spell of "denom" & "*-rat" -> common
refactor '{sub,div}-rat' using '{neg,inv}-rat'
ex 2.2 - done
ex 2.3 - impl of perimeter & area
ex 2.3 - done
2.1.2 - some important points
2.1.3 - done
ex 2.4 - done
impl "run-test" flag & template

so that code can be used directly by "load" without
invoking its testing part
note template should move to the sub-dir or it will not work
cancel "tp"

It doesn't work, I should first find if there's any call to distinguish
 "be called" from "run directly"
ex 2.5 - done

use rm-factor to remove factors from "2^a * 3^b" and count times
ex 2.6 - zero one two

simply apply the rule of add-1 will do
ex 2.6 - impl add
ex 2.6 - done

impl fibonacci numbers based on Church numerial operations
I don't know how to tell if two procedures are identical,
which is impossible actually, so the argument is a normal number :)
2.1.4 - done

note we need to implement "upper-bound" and "lower-bound"
or the code here would not work, see impl in "exercise_2_7"
ex 2.7 - done
ex 2.8 - done
ex 2.9 - impl width