circuit simulate - before `a sample simluation`

look at how many undefined things remained
and how much confusion it has caused
3.3.4 - a sample simulation

go back and complete the code here
although it's 100% not working
circuit simulate - merging code
circuit simulate - last mystery to solve
circuit simulate - still working ...
circuit-simulate - done
circuit simulate - fix typo
3.3.4 - a sample simulation - at least it works
slight change on module load
ex 3.31 - done
ex 3.28 - verified, done
ex 3.29 - bug fix and verified
ex 3.30 - ripple-carry-adder verified
circuit simulate - code rearrangement
ex 3.32 - done
3.3.5 - propagation of constraints
using the constraint system

now let's keep a list of unimplemented procedures
3.3.5 - impl adder
implementing the constraint system
representing connectors