Universality of Finite-State Automata
Consider the following problem: does the given non-deterministic finite-state automaton A accept all words of given length n? The automaton is presented as A = (Σ, Q, I, F, δ), where Σ = {a1, . . . , ak } is the alphabet, Q = {q1, . . . , ql } is the set of states, I ⊆ Q is the set of initial states, F ⊆ Q is the set of final states, and δ ⊆ Q × Σ × Q is the set of transitions. The automaton accepts a word w if there exists a run of A on w that begins in a state from I and ends in a state from F. Write a program that reads the automaton and the number n from standard input (in the format described below), translates the problem above to SAT, runs Z3 over it, and writes to standard output either YES, or NO followed by some rejected word (in the format described below).
Input
The first line of the input contains 5 numbers: k (the number of symbols in the alphabet), l (the number of states), l1 (the number of initial states), lf (the number of final states), and m (the number of transitions). The second line contains k distinct words a1, . . . , ak separated by single spaces, representing the symbols of the alphabet. The third line contains l distinct words q1 , . . . , ql separated by single spaces, representing the states of the automaton. The fourth line contains l1 distinct words separated by single spaces, representing the initial states; each of these words occurs in the list of states (the third line). The fifth line contains lf distinct words separated by single spaces, representing the final states; each of these words occurs in the list of states (the third line). The next m lines represent the transitions. A transition is represented as p a q. Such a transition can be executed if the current state of the automaton is p, and the current symbol is a. The automaton changes its state to q, and moves to the next symbol. For each transition, the words p and q appear in the list of states (the third line), and the word a appears in the list of symbols (the second line). The last line of the input contains a single number n, representing the length of input words.
Output
If all words of length n are accepting, the first and only line of the output should contain YES. Otherwise, the first line should contain NO, the second line should be empty, and the third line should contain an input word of length n rejected by the automaton. The rejected input word should be represented as a sequence of n symbols from the alphabet, separated by single spaces.
Example
For the input data:
2 4 1 1 6
a b
i a b f
i
f
i a i
i b i
i a a
i b b
a a f
b b f
3
a correct result is:
NO
a b a
Allowed languages
We recommand that you write your solution in Python.