code: func (obj *object) Diff(other Object) Object {
  	r := NewObject()
  	obj.Foreach(func(k, v *Term) {
  		if other.Get(k) == nil {
  			r.Insert(k, v)
  		}
  	})
  	return r
  }
docs: // Diff returns a new Object that contains only the key/value pairs that exist in obj.