func (set *StringSet) Insert(s string) {
	if !set.Contains(s) {
		set.elements.Push(s)
	}
}
Insert adds s to the set, so that Contains(s) will now be true if it wasn't
already.
package set
import (
	"container/vector"
)
type StringSet struct {
	elements vector.StringVector
}
func (set *StringSet) Contains(s string) bool {
	for val := range set.elements.Iter() {
		if val == s {
			return true
		}
	}
	return false
}