function! RelativePath(...)
  let path = a:0 ? a:1 : expand('%:p')
  let sep = '/'
  let cwd = split(getcwd(), sep)
  let path_l = split(fnamemodify(path, ':p'), sep)
  while 1
    if empty(cwd) || empty(path_l) || path_l[0] != cwd[0]
      break
    endif
    call remove(path_l, 0)
    call remove(cwd, 0)
  endwhile
  return repeat('..'.sep, len(cwd)).join(path_l, sep)
endfunction