public static void register(CompletionContributor contributor) {
    MapKeysCompletionProvider provider = new MapKeysCompletionProvider();

    contributor.extend(CompletionType.BASIC, PlatformPatterns.psiElement().withParent(PlatformPatterns.psiElement(GrReferenceExpression.class)), provider);
  }
public static void register(CompletionContributor contributor)