code: func (f *Flex) AddItem(item Primitive, fixedSize, proportion int, focus bool) *Flex {
  	f.items = append(f.items, &flexItem{Item: item, FixedSize: fixedSize, Proportion: proportion, Focus: focus})
  	return f
  }
docs: // AddItem adds a new item to the container. The "fixedSize" argument is a width
  // or height that may not be changed by the layout algorithm. A value of 0 means
  // that its size is flexible and may be changed. The "proportion" argument
  // defines the relative size of the item compared to other flexible-size items.
  // For example, items with a proportion of 2 will be twice as large as items
  // with a proportion of 1. The proportion must be at least 1 if fixedSize == 0
  // (ignored otherwise).
  //
  // If "focus" is set to true, the item will receive focus when the Flex
  // primitive receives focus. If multiple items have the "focus" flag set to
  // true, the first one will receive focus.
  //
  // You can provide a nil value for the primitive. This will still consume screen
  // space but nothing will be drawn.