def verify(template)
      unless template.kind_of?(Template)
        raise ArgumentError, 'you can only push Osheet::Template objs to the template set'
      end
      key = template_key(template)
      self[key.first] ||= {}
      self[key.first][key.last] ||= nil
      key
    end
def verify(template)
      unless template.kind_of?(Template)
        raise ArgumentError, 'you can only push Osheet::Template objs to the template set'
      end
      key = template_key(template)
      self[key.first] ||= {}
      self[key.first][key.last] ||= nil
      key
    end
verify the template, init the key set, and return the key string
  otherwise ArgumentError it up