function! CleverTab()
   " strpart(string, start, length)
   "if strpart( getline('.'), 0, col('.')-1 ) =~ '^\s*$'
   if strpart( getline('.'), col('.')-2, 1 ) =~ '^\s*$'
      return "\<Tab>"
   else
      return "\<C-P>"
endfunction
inoremap <Tab> <C-R>=CleverTab()<CR>