Challenge 3: Why Querying and Calling Are Fundamentally Different — Possible Solution
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// Calling a Haskell function, per haskell1-1's own function-first
// model, always produces exactly ONE result (or, for an IO action,
// performs exactly one sequence of effects and returns exactly one
// value). A Haskell function's type signature -- Int -> Int, for
// instance -- promises a single Int comes back for a single Int
// going in. There is no built-in language concept of a function call
// having "more than one right answer" that the caller might want to
// see all of, one at a time.
//
// Querying a Prolog database is structurally different: a query like
// parent(tom, X) isn't asking for "the" value of X -- it's asking
// Prolog to search its ENTIRE database for every possible way X could
// make the statement true. If the database contains two facts that
// both satisfy the query, BOTH are genuinely valid answers, and
// Prolog doesn't have to pick just one. Instead, it returns the first
// one it finds and pauses, offering the programmer a real choice
// (via ;) to ask for the next one, or to stop there. If a query
// happens to have zero matching facts, Prolog reports `false.`
// instead -- again, not an error, just an honest "no, nothing in the
// database makes this true."
//
// The core difference: a Haskell function call is a computation that
// commits to producing one result. A Prolog query is a SEARCH that
// may turn up zero, one, or many results, and the language itself is
// built around letting the caller ask for more of them on demand,
// rather than forcing a single answer to be chosen up front.

WHY THIS WORKS AS AN ANSWER
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This correctly identifies the structural difference the chapter
introduces -- a function call commits to one result, a query searches
for however many results genuinely exist -- and explicitly addresses
the multiple-answer case, including what happens with zero matches,
matching the chapter's own framing.