Make kleene() throw an error if the state is not quantifiable

nfa.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"fmt"
 	"slices"
 )
 
@@ -268,7 +269,11 @@ func concatenate(s1 *State, s2 *State) *State {
 	return s1
 }
 
-func kleene(s1 State) *State {
+func kleene(s1 State) (*State, error) {
+	if s1.isEmpty && s1.assert != NONE {
+		return nil, fmt.Errorf("previous token is not quantifiable")
+	}
+
 	toReturn := &State{}
 	toReturn.transitions = make(map[int][]*State)
 	toReturn.content = newContents(EPSILON)
@@ -283,7 +288,7 @@ func kleene(s1 State) *State {
 	for _, c := range s1.content {
 		toReturn.transitions[c], _ = unique_append(toReturn.transitions[c], &s1)
 	}
-	return toReturn
+	return toReturn, nil
 }
 
 func alternate(s1 *State, s2 *State) *State {