Collections of our design principles

[FujiZ]

1. New-style interfaces (e.g. cq_ex, qp_ex) are first-class citizens; while providing compitibility for legacy interfaces through the same API.

[FujiZ]

2. Check some invariants of the API as much as possible as long as it does not introduce any overhead (e.g. CPU, memory).

[FujiZ]

3. Optimistic error checking: do not introduce overhead on the successful path; report these errors only when the error truly occurs, although this may lose some infomation of the first scene.