Implement ch3.3.4's examples (in progress)
Answer ex3.28
Merge branch 'master' of github.com:kaznum/SICP
Answer ex3.29
Answer ex3.30
Construct recursion reversely than before in ex3.30
Implement ch3.3.4's examples (in progress)
Set the initial signal in advance

When add-action! is called, the action to be added is immediately run.
So set the initial signal in advance.
This explanation is in the following section and ex3.31

This initial condition is very important , so it should be
explained before this exercise :(
Implement ch3.3.4's examples (in progress)