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: encoder.go
    path of file: ./repos/awless/vendor/github.com/boombuler/barcode/code93
    
    the code of the file until where you have to start completion: // Package code93 can create Code93 barcodes
package code93

import (
	"errors"
	"strings"

	"github.com/boombuler/barcode"
	"github.com/boombuler/barcode/utils"
)

type e
    
func prepare(content string) (string, error) {
	result := ""
	for _, r := range content {
		if r > 127 {
			return "", errors.New("Only ASCII strings can be encoded")
		}
		result += extendedTable[int(r)]
	}
	return result, nil
}