code: func (b *Box) FillWithMargins(container *Box, margins int) {
  	b.Fill(container)
  	b.frame.X += margins
  	b.frame.Y += margins
  	b.frame.W -= margins * 2
  	b.frame.H -= margins * 2
  }
docs: // Place a Box inside a given container, with the given margins.
  // Margins are given in number of characters.