As suggested in the to-do here, I suggest to check that an object of type Interval is, indeed, and interval, not only for debugging, i.e., x<y (or, more generally, <(x,y) != 0; theoretically, this naturally generalizes to the many-valued case thanks to the third axiom of many-valued linear orders, i.e., <(x,x) = 0). Hence, I simply suggest removing comment on line 132 and deleting line 131.