def group(group_string)
      raise ArgumentError, "Expected String, got #{group_string.class}." unless group_string.is_a?(String)
      found = Array.new
      children.each do |child|
        found << child if child.tag.group == group_string.upcase
      end
      return found
    end
def group(group_string)
      raise ArgumentError, "Expected String, got #{group_string.class}." unless group_string.is_a?(String)
      found = Array.new
      children.each do |child|
        found << child if child.tag.group == group_string.upcase
      end
      return found
    end
Returns an array of all child elements that belongs to the specified group.

 @param [String] group_string a group string (the first 4 characters of a tag string)
 @return [Array<Element, Item, Sequence>] the matching child elements (an empty array if no children matches)