private static Collection<String> suggestKeywords(PsiElement position) {
    TextRange posRange = position.getTextRange();
    BnfFile posFile = (BnfFile) position.getContainingFile();
    BnfRule statement = PsiTreeUtil.getTopmostParentOfType(position, BnfRule.class);
    final TextRange range;
    if (statement != null) {
      range = new TextRange(statement.getTextRange().getStartOffset(), posRange.getStartOffset());
    } else {
      int offset = posRange.getStartOffset();
      for (PsiElement cur = GrammarUtil.getDummyAwarePrevSibling(position);
          cur != null;
          cur = GrammarUtil.getDummyAwarePrevSibling(cur)) {
        if (cur instanceof BnfAttrs) offset = cur.getTextRange().getEndOffset();
        else if (cur instanceof BnfRule) offset = cur.getTextRange().getStartOffset();
        else continue;
        break;
      }
      range = new TextRange(offset, posRange.getStartOffset());
    }
    final String text =
        range.isEmpty()
            ? CompletionInitializationContext.DUMMY_IDENTIFIER
            : range.substring(posFile.getText());

    PsiFile file =
        PsiFileFactory.getInstance(posFile.getProject())
            .createFileFromText("a.bnf", BnfLanguage.INSTANCE, text, true, false);
    int completionOffset = posRange.getStartOffset() - range.getStartOffset();
    GeneratedParserUtilBase.CompletionState state =
        new GeneratedParserUtilBase.CompletionState(completionOffset) {
          @Override
          public String convertItem(Object o) {
            // we do not have other keywords
            return o instanceof String ? (String) o : null;
          }
        };
    file.putUserData(GeneratedParserUtilBase.COMPLETION_STATE_KEY, state);
    TreeUtil.ensureParsed(file.getNode());
    return state.items;
  }
  @NotNull
  @Override
  public FoldingDescriptor[] buildFoldRegions(
      @NotNull PsiElement root, @NotNull Document document, boolean quick) {
    if (!(root instanceof BnfFile)) return FoldingDescriptor.EMPTY;
    BnfFile file = (BnfFile) root;

    final ArrayList<FoldingDescriptor> result = new ArrayList<FoldingDescriptor>();
    for (BnfAttrs attrs : file.getAttributes()) {
      TextRange textRange = attrs.getTextRange();
      if (textRange.getLength() <= 2) continue;
      result.add(new FoldingDescriptor(attrs, textRange));
    }
    for (BnfRule rule : file.getRules()) {
      // result.add(new FoldingDescriptor(rule, rule.getTextRange()));
      BnfAttrs attrs = rule.getAttrs();
      if (attrs != null) {
        result.add(new FoldingDescriptor(attrs, attrs.getTextRange()));
      }
    }
    if (!quick) {
      PsiTreeUtil.processElements(
          file,
          new PsiElementProcessor() {
            @Override
            public boolean execute(@NotNull PsiElement element) {
              if (element.getNode().getElementType() == BnfParserDefinition.BNF_BLOCK_COMMENT) {
                result.add(new FoldingDescriptor(element, element.getTextRange()));
              }
              return true;
            }
          });
    }

    return result.toArray(new FoldingDescriptor[result.size()]);
  }