I believe there should be some fallacies in test cases since the description require a minimum D.
Other than the case raised by tchar. Also there can be test case like:
[[2/4]]
which should generate this output:
[[1/2]]

Maybe the author could modify the problem description so that solvers are required to put the l.c.m. as the denominator.

Agreed. This solution is indeed clean and nicely organized. However if performance is to be considered, this solution is not as good as others are.