def keys
      @hash.deep.each_with_object([]) do |(key_path, value), result|
        result << key_path.to_key_path unless value.is_a?(Hash)
      end
    end
def keys
      @hash.deep.each_with_object([]) do |(key_path, value), result|
        result << key_path.to_key_path unless value.is_a?(Hash)
      end
    end
Return all maximal key paths in a tree

 :call-seq:
   keys => Array of KeyTree::Path