vim - toggle between relative / absolute / no line numbering

gistfile1.vim

if exists("+relativenumber")
  set relativenumber  " show relative line numbers
  set numberwidth=3   " narrow number column
  " cycles between relative / absolute / no numbering
  function! RelativeNumberToggle()
    if (&relativenumber == 1)
      set number number?
    elseif (&number == 1)
      set nonumber number?
    else
      set relativenumber relativenumber?
    endif
  endfunc
  nnoremap <silent> <leader>n :call RelativeNumberToggle()<CR>
else                  " fallback
  set number          " show line numbers
  " inverts numbering
  nnoremap <silent> <leader>n :set number! number?<CR>
endif