For deterministic functions/methods, the same set of inputs should 
always produces the same output. What about adding something like:

  assert_follows_spec(method_or_class_name, spec)

Where spec is an array of input and output pairs (and optionally an 
exception, if the input should not be accepted):

  [{input => [1, 1, 1], output => 1},
   {input => [8, 1, 2], output => 4},
   {input => [8, 3, 4], output => 6},
   {input => [4, 3, 2], output => 6},
   {input => [1, 2, nil], exception => ArgumentError},
   {input => [1, 2, 0], exception => ZeroDivisionError}]

This can make unit tests shorter and more readable. We can transform the 
spec into an HTML table, include it into the API documentation, etc.

The assert_follows_spec() could even perform several iterations of tests 
if wanted, to test deterministic-ness.

-- 
dave