== Expressing integers using four nines

What non-negative integers can you write as expressions containing exactly
four occurrences the number 9, and any of the binary operators *, /, %, +,
-, prefix negations, and any number of matching pairs of parentheses you
care to use?

The program should accept an upper limit N as a command-line argument.
It should then print all integers 0..N in increasing order, along with
an expression with four nines, if any such was found.

More precisely, let's say your program didn't find a four-nines expression
for the number 18. Then it should simply print

    18

on that particular line. If it *did* find a number, it should include the
expression it found, like this:

    18 = 9 + 9 * (9 / 9)