Vim Mapkey() function: an inverse for maparg()

vim_mapkey.vim

function! Mapkey (keys, mode)
    " Inverse of maparg()
    " Pass in a key sequence and the first letter of a Vim mode.
    " Returns key mapping mapped to it in that mode, else '' if none.
    " <Keys> - e.g. "<Space>" and "<CR>" - are matched case-insensitively,
    " as are things before dashes, e.g. the "C-S-" part of "<C-S-A>".
    " Chorded "<keys>" are not handled properly yet (i.e. "<C-Space>")
    " Example:
    "   :nnoremap <Tab> :bn<CR>
    "   :call Mapkey(':bn<CR>', 'n')
    "   " returns <Tab>
    redir => mappings | silent! map | redir END
    let keys = substitute(a:keys, '<\(\w*\|.*-\)\=\(\w*\)\=>', '<\L\1\e\2>', 'g')
    for map in split(mappings, '\n')
        let seq = matchstr(map, '\s\+\zs\S*')
        if substitute(maparg(seq,a:mode), '<\(\w*\|.*-\)\=\(\w*\)\=>', '<\L\1\e\2>', 'g') == keys
            return seq
        endif
    endfor
    return ''
endfunction