public Set<String> getRequiredFiles(OperatingSystem os) {
    Set neededFiles = new HashSet();

    for (Library library : getRelevantLibraries()) {
      if (library.getNatives() != null) {
        String natives = (String)library.getNatives().get(os);
        if (natives != null) neededFiles.add("launcher/" + library.getArtifactPath(natives)); 
      }
      else { neededFiles.add("launcher/" + library.getArtifactPath()); }

    }

    return neededFiles;
  }