datastore/query.go
func (q *Query) Start(c Cursor) *Query {
	q = q.clone()
	if c.cc == nil {
		q.err = errors.New("datastore: invalid cursor")
		return q
	}
	q.start = c.cc
	return q
}
func (q *Query) Start(c Cursor) *Query {
	q = q.clone()
	if c.cc == nil {
		q.err = errors.New("datastore: invalid cursor")
		return q
	}
	q.start = c.cc
	return q
}
// Start returns a derivative query with the given start point.