func (os *orderedSet) append(s string) {
	if _, ok := os.included[s]; !ok {
		os.list = append(os.list, s)
		os.included[s] = struct{}{}
	}
}
func (os *orderedSet) append(s string) {
	if _, ok := os.included[s]; !ok {
		os.list = append(os.list, s)
		os.included[s] = struct{}{}
	}
}
func ( os * orderedSet ) append ( s string ) { if _ , ok := os . included [ s ] ; ! ok { os . list = append ( os . list , s ) os . included [ s ] = struct { } { } } }
// append adds s to the end of os, only if it is not included already.
append adds s to the end of os only if it is not included already .