func (s Nodes) Add(n graph.Node) {
	s[n.ID()] = n
}
The simple accessor methods for Nodes are provided to allow ease of
implementation change should the need arise.
Add inserts an element into the set.
package set
import "gonum.org/v1/gonum/graph"
type Ints map[int]struct{}
func (s Ints) Add(e int) {
	s[e] = struct{}{}
}
func (s Ints) Has(e int) bool {
	_, ok := s[e]
	return ok
}
func (s Ints) Remove(e int) {
	delete(s, e)
}
func (s Ints) Count() int {
	return len(s)
}
func IntsEqual(a, b Ints) bool {
	if intsSame(a, b) {
		return true
	}
	if len(a) != len(b) {
		return false
	}
	for e := range a {
		if _, ok := b[e]; !ok {
			return false
		}
	}
	return true
}
type Int64s map[int64]struct{}
func (s Int64s) Add(e int64) {
	s[e] = struct{}{}
}
func (s Int64s) Has(e int64) bool {
	_, ok := s[e]
	return ok
}
func (s Int64s) Remove(e int64) {
	delete(s, e)
}
func (s Int64s) Count() int {
	return len(s)
}
func Int64sEqual(a, b Int64s) bool {
	if int64sSame(a, b) {
		return true
	}
	if len(a) != len(b) {
		return false
	}
	for e := range a {
		if _, ok := b[e]; !ok {
			return false
		}
	}
	return true
}
type Nodes map[int64]graph.Node
func NewNodes() Nodes {
	return make(Nodes)
}
func NewNodesSize(n int) Nodes {
	return make(Nodes, n)
}