def each
      return to_enum unless block_given?
      @index.each { |name, attribute| yield attribute if name.kind_of?(Symbol) }
      self
    end
def each
      return to_enum unless block_given?
      @index.each { |name, attribute| yield attribute if name.kind_of?(Symbol) }
      self
    end
Initialize an AttributeSet

 @param [AttributeSet] parent
 @param [Array] attributes

 @return [undefined]

 @api private
 Iterate over each attribute in the set

 @example
   attribute_set = AttributeSet.new(attributes, parent)
   attribute_set.each { |attribute| ... }

 @yield [attribute]

 @yieldparam [Attribute] attribute
   each attribute in the set

 @return [self]

 @api public