def step_dfa(D, q, c):
    """In : D (consistent DFA)
            q (state in D)
            c (symbol in D's sigma)
       Out: next state of q via c (state in D) 
    """
    assert(c in D["Sigma"]), "step_dfa given c not in Sigma."
    assert(q in D["Q"]), "step_dfa given q not in Q."
    return D["Delta"][(q,c)]