Function epsilon_closure 

pub(crate) fn epsilon_closure(
    nfa: &NFA,
    start_nfa_id: StateID,
    look_have: LookSet,
    stack: &mut Vec<StateID>,
    set: &mut SparseSet,
)

Compute the epsilon closure for the given NFA state. The epsilon closure consists of all NFA state IDs, including start_nfa_id, that can be reached from start_nfa_id without consuming any input. These state IDs are written to set in the order they are visited, but only if they are not already in set. start_nfa_id must be a valid state ID for the NFA given.

look_have consists of the satisfied assertions at the current position. For conditional look-around epsilon transitions, these are only followed if they are satisfied by look_have.

stack must have length 0. It is used as scratch space for depth first traversal. After returning, it is guaranteed that stack will have length 0.