docs: @Requires("kind != null")
    @Ensures({
      "result != null",
      "!result.contains(null)"
    })
    List<ClassContractHandle> getClassHandles(ContractKind kind) {
      ArrayList<ClassContractHandle> matched =
          new ArrayList<ClassContractHandle>();
      for (ClassContractHandle h : classHandles) {
        if (kind.equals(h.getKind())) {
          matched.add(h);
        }
      }
      return matched;
    }