code: func (f *Fpdf) AddSpotColor(nameStr string, c, m, y, k byte) {
  	if f.err == nil {
  		_, ok := f.spotColorMap[nameStr]
  		if !ok {
  			id := len(f.spotColorMap) + 1
  			f.spotColorMap[nameStr] = spotColorType{
  				id: id,
  				val: cmykColorType{
  					c: byteBound(c),
  					m: byteBound(m),
  					y: byteBound(y),
  					k: byteBound(k),
  				},
  			}
  		} else {
  			f.err = fmt.Errorf("name \"%s\" is already associated with a spot color", nameStr)
  		}
  	}
  }
docs: // AddSpotColor adds an ink-based CMYK color to the gofpdf instance and
  // associates it with the specified name. The individual components specify
  // percentages ranging from 0 to 100. Values above this are quietly capped to
  // 100. An error occurs if the specified name is already associated with a
  // color.