//find the expression that corresponds to the question word
func findExpr(nextQn *rx.BitSet, e []*RegEx) string {
b := new(rx.BitSet)
for i := 0; i < len(e); i++ {
b.Set(e[i].Index)
if b.Equals(nextQn) {
return e[i].Rexpr
}
b.Clear(e[i].Index)
}
return ""
}
