/**
   * Creates abstract version of each method in each class (does not touch method itself as opposite
   * to {@link #moveMethods(com.jetbrains.python.psi.PyClass, java.util.Collection,
   * com.jetbrains.python.psi.PyClass...)})
   *
   * @param currentFunctions functions to make them abstract
   * @param to classes where abstract method should be created
   */
  private static void makeMethodsAbstract(
      final Collection<PyFunction> currentFunctions, final PyClass... to) {
    final Set<PsiFile> filesToCheckImport = new HashSet<PsiFile>();
    final Set<PyClass> classesToAddMetaAbc = new HashSet<PyClass>();

    for (final PyFunction function : currentFunctions) {
      for (final PyClass destClass : to) {
        final PyFunctionBuilder functionBuilder =
            PyFunctionBuilder.copySignature(function, DECORATORS_MAY_BE_COPIED_TO_ABSTRACT);
        functionBuilder.decorate(PyNames.ABSTRACTMETHOD);
        final LanguageLevel level = LanguageLevel.forElement(destClass);
        PyClassRefactoringUtil.addMethods(
            destClass, false, functionBuilder.buildFunction(destClass.getProject(), level));
        classesToAddMetaAbc.add(destClass);
      }
    }

    // Add ABCMeta to new classes if needed
    for (final PyClass aClass : classesToAddMetaAbc) {
      if (addMetaAbcIfNeeded(aClass)) {
        filesToCheckImport.add(aClass.getContainingFile());
      }
    }

    // Add imports for ABC if needed
    for (final PsiFile file : filesToCheckImport) {
      addImportFromAbc(file, PyNames.ABSTRACTMETHOD);
      addImportFromAbc(file, ABC_META_CLASS);
      PyClassRefactoringUtil.optimizeImports(file); // To remove redundant imports
    }
  }