code: func updateSetExpToString(cfName string, val interface{} /*, opts Options*/) (
  	statement string, fields []string, err error) {
  
  	fields = sliceOfFieldNames(val)
  
  	statement = updateStatement(cfName, fields)
  	return statement, fields, nil
  }
docs: // updateSetExpToString generates UPDATE + SET part of SQL statement
  // for fields of an entity