code: def append(self, value, key=''):
          
          if type(value) in (list, tuple, dict):
              if type(value)==dict:
                  for k in value.keys():
                      self.append(value[k], k)
                  return value.keys()
              keys = []
              for child in value:
                  keys.append( self.append(child) )
              return keys
          
          key = str(key)
          if not isinstance(value, Widget):
              raise ValueError('value should be a Widget (otherwise use add_child(key,other)')
  
          if 'left' in value.style.keys():
              del value.style['left']
          if 'right' in value.style.keys():
              del value.style['right']
  
          if not 'order' in value.style.keys():
              value.style.update({'position':'static', 'order':'-1'})
  
          if key.isdigit():
              value.style['order'] = key
  
          key = value.identifier if key == '' else key
          self.add_child(key, value)
  
          return key
docs: It allows to add child widgets to this.
          The key allows to access the specific child in this way widget.children[key].
          The key have to be numeric and determines the children order in the layout.
  
          Args:
              value (Widget): Child instance to be appended.
              key (str): Unique identifier for the child. If key.isdigit()==True '0' '1'.. the value determines the order
              in the layout