docs: public static <U, P> FibonacciHeap<U, P> merge(FibonacciHeap<U, P> a, FibonacciHeap<U, P> b) {
          Preconditions.checkArgument(a.comparator().equals(b.comparator()),
                  "Heaps that use different comparators can't be merged.");
  
          final FibonacciHeap<U, P> result = FibonacciHeap.create(a.comparator);
          result.oMinEntry = a.mergeLists(a.oMinEntry, b.oMinEntry);
          result.size = a.size + b.size;
  
          return result;
      }