= Solution
If `f : 'a -> 'b`, then the success constructor applied to `f x` has type `'b result`, and the exception alternative has that same type. Therefore
``
what : ('a -> 'b) -> 'a -> 'b result
``
The zero comparison fixes the labels to type `int`, and `tony` with this predicate accepts an `int tree` and normally produces a Boolean. The requested expression consequently has type
``
int tree -> bool result
``
so \b[the wrapper maps an integer tree to a packaged Boolean result or exception packet].
For `ta`, the packet `Ian true` means that a normal successful search occurred: `ta` has at least one root-to-`Lf` path all of whose branch labels are nonzero. It can be `Lf`; if nonempty, its root must be nonzero, but labels away from the successful path need not be. For example, `Br(1,Lf,Br(0,Lf,Lf))` succeeds despite containing a zero.
For `tb`, `Cherie Blair` means that the search raised the designated exception. Every root-to-`Lf` path has a zero-labeled branch somewhere. Thus `tb` is nonempty, but its root need not be zero: `Br(1,Br(0,Lf,Lf),Br(0,Lf,Lf))` fails. With the supplied total comparison predicate, `tony` can return only `true` or raise `Blair`, never return `false`. \b[`ta` admits an all-nonzero path; `tb` admits none.]
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