Returns the result of the first successful application of a function to
the elements of a list.
DESCRIPTION
tryfind f [x1,...,xn] returns (f xi) for the first xi in the list
for which application of f does not raise an exception. However, if
Interrupt is raised in the course of some application of f xi, then
tryfind f [x1,...,xn] raises Interrupt.
FAILURE
Fails if the application of f fails for all elements in the list. This
will always be the case if the list is empty.