Missing high-level plan of optimization

It seems that there is lacking a normative reference of overall optimizations, and by which criteria an issue related to the optimization can be closed.

For example, #114 needs to do the optimization of a kind of local CSE. CSE is a common and general class of classical optimizations. When should such optimization class be involved?

Note this is mostly a doc issue, parallel to #81.

Now we need to find out how to optimize this:

Closing as a duplicate of the other.