Function select_transitions 

pub fn select_transitions<F: FnMut(TransIdx) -> bool>(
    chart: &CompiledChart,
    config: &StateSet,
    history: &History,
    event: Option<&str>,
    cond: F,
) -> Vec<TransIdx>

selectTransitions (with Some(event)) and selectEventlessTransitions (with None): visit the active atomic states in document order and, for each, take the first transition in document order on the state or its nearest ancestor that matches the event and whose condition holds; then remove conflicts. cond is called, in this order, for each candidate transition that has a condition, and answers whether it holds.