undefined: retrieve

When run `e:\code\github\gwp\Chapter_7_Creating_Web_Services\web_service>go run server.go` got an error:
```
# command-line-arguments
.\server.go:50:15: undefined: retrieve
.\server.go:71:12: post.create undefined (type Post has no field or method create)
.\server.go:86:15: undefined: retrieve
.\server.go:109:15: undefined: retrieve
```