func (e *extensionTagMap) lookup(tagInfo uint64) (handleTagDecFn, error) {
	fn, ok := extensionTagDec[tagInfo]
	if !ok {
		return nil, fmt.Errorf(
			"0x%x not matched as registered tag extension handler", tagInfo)
	}
	return fn, nil
}
Look for a function registered to handle a given tag info
package cbor
import (
	"fmt"
	"math/big"
	"reflect"
)
type handleTagDecFn func(*Decoder, interface{}) error
type extensionTagMap map[uint64]handleTagDecFn
var extensionTagDec extensionTagMap = make(extensionTagMap)
func (e *extensionTagMap) register(tagInfo uint64, fn handleTagDecFn) error {
	if _, ok := extensionTagDec[tagInfo]; ok {
		return fmt.Errorf("0x%x tag information is already registered", tagInfo)
	}
	extensionTagDec[tagInfo] = fn
	return nil
}