Have your test and not write it too. Very appealing, but one problem I've found with this general idea is equivalent but non-identical results. A simply-solved

Have your test and not write it too. Very appealing, but one problem I've found with this general idea is equivalent but non-identical results. A simply-solved example is a serialized set: different orderings differ, but are equivalent. You can get more complex ones, such as ASTs ax+bx and (a+b)x. Such cases should be pretty rare, but they come up for me all the time.