Looking at 2.3.1 I tested the core library example of the document on playground with the following frame

{
  "@context": {
    "@vocab": "https://example.org/"
  },
  "@graph" : [
      {
        "@type": "Library",
        "contains": {
            "@type": "Book",
            "@embed": "@never"
        }
      },
      {
        "@type": "Book",
        "contains": {
            "@type": "Chapter",
            "@embed": "@last"
        }
      }
  ]
}

And I got the reply that a frame document must have only one frame object.

O.k., playground is playing per spec, but I was a bit surprised: why can't I have a frame like the one above? In the result, the 'library' would just list the books by ID, and the rest of the graph would include each book separately, including the relevant chapters. Isn't that a viable use case?