You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: evaluator.go
    path of file: ./repos/docker-ce/components/engine/builder/dockerfile
    
    the code of the file until where you have to start completion: // Package dockerfile is the evaluation step in the Dockerfile parse/evaluate pipeline.
//
// It incorporates a dispatch table based on t
    
func (r *stagesBuildResults) validateIndex(i int) error {
	if i == len(r.flat) {
		return errors.New("refers to current build stage")
	}
	if i < 0 || i > len(r.flat) {
		return errors.New("index out of bounds")
	}
	return nil
}