func (t *Tree) add() {
	tree := t.treeSet[t.Name]
	if tree == nil || IsEmptyTree(tree.Root) {
		t.treeSet[t.Name] = t
		return
	}
	if !IsEmptyTree(t.Root) {
		t.errorf("template: multiple definition of template %q", t.Name)
	}
}

// IsEmptyTree reports whether this tree (node) is empty of everything but space or comments.
func IsEmptyTree(n Node) bool {
	switch n := n.(type) {
	case nil:
		return true

}

func TestIsEmpty(t *testing.T) {
	if !IsEmptyTree(nil) {
		t.Errorf("nil tree is not empty")
	}
	for _, test := range isEmptyTests {
		tree, err := New("root").Parse(test.input, "", "", make(map[string]*Tree), nil)
		if err != nil {
			t.Errorf("%q: unexpected error: %v", test.name, err)
			continue
		}
		if empty := IsEmptyTree(tree.Root); empty != test.empty {