only kFinalSymobl arcs enter final state

[qindazhu]

No description provided.

[danpovey]

We can put something like assert(IsValid(input_fsa)) on other operations. For operations that have other requirements, e.g. top-sortedness, can put assert(IsValid(..) && IsTopSorted(..)).

[danpovey]

And, guys, please always be absolutely clear in your head and in the functions' documentation, what the pre-conditions and post-conditions of functions are. There can also be conditional ones, e.g. "Input must be valid; if the input is top-sorted the output will be top-sorted". These kinds of things can also be checked via asserts.

[csukuangfj]

I will introduce glog in my pending pullrequest to check preconditions.