API call to get document properties and the checked out to display name and email

[ra1449]

I'm using /_api/web/lists/getbytitle('myLibrary')/items?$select=CheckoutUserId in an ajax function.  I need the checked out to display name and email instead of the CheckoutUserId.   I tried appending with 

&$expand=CheckoutUserId/Id but get an error response, "The field or property 'CheckoutUserId' does not exist."  

[#2]

I figured it out....
https://yourTenant.sharepoint.com/sites/yourSite/_api/web/lists/getbytitle('yourLibrary')/items?$sel...