1 function! CleverTab()
2     if strpart( getline(‘.‘) ,0,col(‘.‘)-1) =~‘^\s*$‘
3         return "\<Tab>"
4     else    
5         return "\<C-N>"
6     endif   
7 endfunction
8 inoremap <Tab> <C-R>=CleverTab()<CR>

set shortmess=a
set cmdheight=2