private static void test_subtraction(string minuend, string subtrahend, string expected_difference)
{
string actual_difference = subtract_strings(minuend, subtrahend);
System.Diagnostics.Trace.Assert (actual_difference == expected_difference,
"Test failure: "
+ "Subtracting `" + subtrahend
+ "' from `" + minuend
+ "' yielded `" + actual_difference
+ "', but should have yielded `" + expected_difference + "'.");
}