code: func (c *Collection) FieldIndexKey(fieldName string) (string, error) {
  	return c.spec.fieldIndexKey(fieldName)
  }
docs: // FieldIndexKey returns the key for the sorted set used to index the field
  // identified by fieldName. It returns an error if fieldName does not identify a
  // field in the spec or if the field it identifies is not an indexed field.