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: rules.go
    path of file: ./repos/awless/template/params
    
    the code of the file until where you have to start completion: package params

import (
	"errors"
	"fmt"
	"sort"
	"strings"
)

type Rule interface {
	Visit(func(Rule))
	Run(input []string) error
	Required() []string
	Missing(input []string) []string
	String() string
}

func List(r Rule) (required []string, optionals []string, sugge
    
func Opt(i ...interface{}) Rule {
	o := opt{}
	for _, v := range i {
		switch vv := v.(type) {
		case string:
			o.optionals = append(o.optionals, vv)
		case []suggested:
			o.suggested = append(o.suggested, vv...)
		default:
			panic("invalid type for optional param")
		}
	}
	return o
}