" define a function for cycling the tabstops from 2 -> 4 -> 8 -> 2
set ts=4 sw=4
function Toggle_Tab_Width()
    if &ts == 2
        set ts=4 sw=4
    elseif &ts == 4
        set ts=8 sw=8
    else
        set ts=2 sw=2
    endif
endfunction

" call the funciton via the ex style command ':TG<ret>'
command TG call Toggle_Tab_Width()

" set up Function-Key-1 to run the function. <F1> normally
" invokes help, but you can still get to that via :help.
unmap <F1>
map <F1> :TG^M

" Allow this to work in insert mode too.
unmap! <F1>
map! <F1> <esc>:TG^Mli