update

const (
	// DeleteField is used as a value in a call to update or Set with merge to indicate
	// that the corresponding child should be deleted.
	DeleteField sentinel = iota

	// ServerTimestamp is used as a value in a call to update to indicate that the
	// child's value should be set to the time at which the server processed
	// the request.
	//
	// ServerTimestamp must be the value of a fieldName directly; it cannot appear in
	// array or struct values, or in any value that is itself inside an array or
	// struct.
	ServerTimestamp
)