Challenge 3: Calling Process() Directly on the Class Type — Possible Solution
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Using Challenge 2's Document class:

Document doc = new Document();
doc.Process();   // attempting to call directly on the Document-typed variable

Representative compile error:

    Program.cs(9,9): error CS1061: 'Document' does not contain a
    definition for 'Process' and no accessible extension method
    'Process' accepting a first argument of type 'Document' could be
    found (are you missing a using directive or an assembly
    reference?)

Explanation:
Both Process() implementations in Document were written using explicit
interface implementation syntax (IPrintable.Process() and
ISavable.Process()), which means neither one becomes a genuine public
member of the Document class itself -- they exist ONLY as part of
each interface's own contract. The Document type, looked at on its
own, has no member named Process() at all from the compiler's
perspective; that's why the error reads as if Process() doesn't exist
on Document, rather than being ambiguous between two candidates. The
only way to reach either implementation is through a reference or
cast typed as IPrintable or ISavable specifically, exactly as
Challenge 2 demonstrated.

WHY THIS WORKS AS AN ANSWER
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This reproduces the exact CS1061 "does not contain a definition"
error the chapter's warn-box describes, and correctly explains it in
terms of explicitly-implemented members never becoming part of the
concrete class's own public surface.