func (s *Set) Add(element string) bool {
	if s.Exists(element) {
		return false
	}
	s.elements = append(s.elements, element)
	s.index[element] = struct{}{}
	return true
}
Add adds element to set
return false if element already exists
package util
type Set struct {
	elements []string
	index    map[string]struct{}
}
func NewSet() Set {
	return Set{
		elements: []string{},
		index:    map[string]struct{}{},
	}
}