docs: def MakeRanges(codes):
    """Turn a list like [1,2,3,7,8,9] into a range list [[1,3], [7,9]]"""
    ranges = []
    last = -100
    for c in codes:
      if c == last+1:
        ranges[-1][1] = c
      else:
        ranges.append([c, c])
      last = c
    return ranges