function! TerminalTab()
    " find evey terminal buffer
    let b = filter(range(1, bufnr('$')),
                'getbufvar(v:val, "&buftype", "ERROR") == "terminal"')
    " if no terminal buffers are available
    if len(b) == 0
        tabnew
        terminal
    else
        " we only care for the first terminal buffer
        let b = b[0]
        let bb = win_findbuf(b)
        " if there is no window in any tab that contains a terminal
        if len(bb) == 0
            tabnew
            buffer b
        else
            " again we only care for the first match
            let bb = win_id2tabwin(bb[0])
            exe 'tabnext'.bb[0]
            exe bb[1].'wincmd w'
        endif
    endif
    " we open a terminal to do something
    startinsert
endfunc
nmap <silent> <leader>s :call TerminalTab()<cr>