func IsMultiError(err error) bool {
	_, ok := err.(*MultiError)
	return ok
}
IsMultiError returns true if the given error is type *MultiError.
package jws
import (
	"fmt"
	"github.com/SermoDigital/jose/crypto"
)
type VerifyCallback func(JWS) ([]interface{}, error)
func (j *jws) VerifyCallback(fn VerifyCallback, methods []crypto.SigningMethod, o *SigningOpts) error {
	keys, err := fn(j)
	if err != nil {
		return err
	}
	return j.VerifyMulti(keys, methods, o)
}