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"Use TAB to complete when typing words, else inserts TABs as usual.
function! Tab_Or_Complete()
  if col('.')>1 && strpart( getline('.'), col('.')-2, 3 ) =~ '^\w'
    return "\<C-N>"
  else
    return "\<Tab>"
  endif
endfunction
inoremap <Tab> <C-R>=Tab_Or_Complete()<CR>