code: public Stream<LabelledInstance> instances() throws IOException {
          if (single()) {
              return Collections.singletonList(instance(new File(instance))).stream();
          }
  
          @SuppressWarnings("resource")
          Stream<String> s = Files.lines(Paths.get(instances), StandardCharsets.UTF_8);
          return s.map(x -> instance(new File(x)));
      }
docs: List all the instances to solve.
  @return a list of instances
  @throws IOException if it was not possible to get all the instances