code: func (f *Fpdf) SetMargins(left, top, right float64) {
  	f.lMargin = left
  	f.tMargin = top
  	if right < 0 {
  		right = left
  	}
  	f.rMargin = right
  }
docs: // SetMargins defines the left, top and right margins. By default, they equal 1
  // cm. Call this method to change them. If the value of the right margin is
  // less than zero, it is set to the same as the left margin.