func indexType(fields []*Field, typ string) string {
	index := ""
	for range fields {
		index += "map[string]"
	}
	index += typ
	return index
}
func indexType(fields []*Field, typ string) string {
	index := ""
	for range fields {
		index += "map[string]"
	}
	index += typ
	return index
}
// Return an index type of the form "map[string]map[string]...<typ>", with one
// level of indexing for each given field.