def groups
      children.find_all {|child| child.is_a?(Group) }.map do |group|
        group.supergroup = self
        group
      end
    end
def groups
      children.find_all {|child| child.is_a?(Group) }.map do |group|
        group.supergroup = self
        group
      end
    end
@example Return all the sub-groups of the main group

     main_group = Xcode.project('MyProject.xcodeproj').groups
     main_group.groups

 @return [Array] the sub-groups contained within this group.