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: iterator.go
    path of file: ./repos/teller/vendor/google.golang.org/api/iterator
    
    the code of the file until where you have to start completion: // Copyright 2016 Google LLC.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package iterator provides support for standard Google API iterators.
// See https://github.com/GoogleCloudPlatform/gcloud-golang/wiki/Iterator-Guidelines.
package iterator

import (
	"errors"
	"fmt"
	"reflect"
)

// Done is returned by an iterator's Next method when the iteration is
// complete; when there are no more items to return.
var Done = errors.New("no more items in iterator")

// We don't support mixed calls to N
    
func (p *Pager) NextPage(slicep interface{}) (nextPageToken string, err error) {
	p.pageInfo.nextPageCalled = true
	if p.pageInfo.err != nil {
		return "", p.pageInfo.err
	}
	if p.pageInfo.nextCalled {
		p.pageInfo.err = errMixed
		return "", p.pageInfo.err
	}
	if p.pageInfo.bufLen() > 0 {
		return "", errors.New("must call NextPage with an empty buffer")
	}
	// The buffer must be empty here, so takeBuf is a no-op. We call it just to get
	// the buffer's type.
	wantSliceType := reflect.PtrTo(reflect.ValueOf(p.pageInfo.takeBuf()).Type())
	if slicep == nil {
		return "", errors.New("nil passed to Pager.NextPage")
	}
	vslicep := reflect.ValueOf(slicep)
	if vslicep.Type() != wantSliceType {
		return "", fmt.Errorf("slicep should be of type %s, got %T", wantSliceType, slicep)
	}
	for p.pageInfo.bufLen() < p.pageSize {
		if err := p.pageInfo.fill(p.pageSize - p.pageInfo.bufLen()); err != nil {
			p.pageInfo.err = err
			return "", p.pageInfo.err
		}
		if p.pageInfo.Token == "" {
			break
		}
	}
	e := vslicep.Elem()
	e.Set(reflect.AppendSlice(e, reflect.ValueOf(p.pageInfo.takeBuf())))
	return p.pageInfo.Token, nil
}