<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def fast_combine_pairs(files, force_single, full_name, separators):
 """ assume files that need to be paired are within 10 entries of each other, once the list is sorted """

    files = sort_filenames(files)
    chunks = tz.sliding_window(10, files)
    pairs = [combine_pairs(chunk, force_single, full_name, separators) for chunk in chunks]
    pairs = [y for x in pairs for y in x]
    longest = defaultdict(list)
    # for each file, save the longest pair it is in
    for pair in pairs:
        for file in pair:
            if len(longest[file]) < len(pair):
                longest[file] = pair
    # keep only unique pairs
    longest = {tuple(sort_filenames(x)) for x in longest.values()}
    # ensure filenames are R1 followed by R2
    return [sort_filenames(list(x)) for x in longest]