-whatever function `f` we supply it, what is the smallest integer `x`
-where `f x` is `true`. (You may be thinking: but that
-smallest-integer function is not a proper algorithm, since it is not
-guaranteed to halt in any finite amount of time for every argument.
-This is the famous [[!wikipedia Halting problem]]. But the fact that
-an implementation may not terminate doesn't mean that such a function
-isn't well-defined. The point of interest here is that its definition
-requires recursion in the function definition.)
+whatever function `f` we supply it, what is the smallest natural number `x`
+where `f x` is `true` (even if `f` itself is a function we do already know how to define).