pub fn new(lts: &LabelledTransitionSystem) -> IncomingTransitions {

        let mut incoming_transitions: Vec<(LabelIndex, StateIndex)> = vec![(0,0); lts.num_of_transitions()];

        let mut state2incoming: Vec<TransitionIndex> = vec![TransitionIndex::default(); lts.num_of_states()];

        for state_index in lts.iter_states() {

            for (label_index, to) in lts.outgoing_transitions(state_index) {

                state2incoming[*to].end += 1;

                if lts.is_hidden_label(*label_index) {

                    state2incoming[*to].silent += 1;

                }

        state2incoming.iter_mut().fold(0, |count, index| {

            let end = count + index.end;

            index.start = end - index.silent;

            index.end = end;

            index.silent = end;

            end

        });

        for state_index in lts.iter_states() {

            for (label_index, to) in lts.outgoing_transitions(state_index) {

                let index = &mut state2incoming[*to];

                if lts.is_hidden_label(*label_index) {

                    // Place at end of incoming transitions.

                    index.silent -= 1;

                    incoming_transitions[index.silent] = (*label_index, state_index);

                } else {

                    index.start -= 1;

                    incoming_transitions[index.start] = (*label_index, state_index);

                }

        IncomingTransitions { incoming_transitions, state2incoming}

    }

    pub fn incoming_transitions(&self, state_index: usize) -> impl Iterator<Item = &(LabelIndex, StateIndex)> {

        self.incoming_transitions[self.state2incoming[state_index].start .. self.state2incoming[state_index].end].iter()

    }

    pub fn incoming_silent_transitions(&self, state_index: usize) -> impl Iterator<Item = &(LabelIndex, StateIndex)>  {

        self.incoming_transitions[self.state2incoming[state_index].silent .. self.state2incoming[state_index].end].iter()

    }

    fn test_incoming_transitions() {

        let lts = random_lts(10, 3, 3);

        let _ = IncomingTransitions::new(&lts);

    }