When we hit an accept state in either the DFA or the ATN, we have to notify the character stream to start buffering characters via {@link IntStream#mark} and record the current state. The current sim state includes the current index into the input, the current line, and current character position in that line. Note that the Lexer is tracking the starting line and characterization of the token. These variables track the "state" of the simulator when it hits an accept state.