code: func (wp WeightedPages) Prev(cur Page) Page {
  	for x, c := range wp {
  		if c.Page == cur {
  			if x == 0 {
  				return wp[len(wp)-1].Page
  			}
  			return wp[x-1].Page
  		}
  	}
  	return nil
  }
docs: // Prev returns the previous Page relative to the given Page in
  // this weighted page set.