docs: def check_valid_hierarchies(instance):
      """ Additional check for the hierarchies model, to ensure that levels
      given are pointing to actual dimensions """
      hierarchies = instance.get('hierarchies', {}).values()
      dimensions = set(instance.get('dimensions', {}).keys())
      all_levels = set()
      for hierarcy in hierarchies:
          levels = set(hierarcy.get('levels', []))
          if len(all_levels.intersection(levels)) > 0:
              # Dimension appears in two different hierarchies
              return False
          all_levels = all_levels.union(levels)
          if not dimensions.issuperset(levels):
              # Level which is not in a dimension
              return False
      return True