code: def _combine_ranges(ranges):
      
      ranges2d = []
      for irow, rowranges in enumerate(ranges):
          ja = 0
          jb = 0
          while jb < len(rowranges):
              bcol0, bcol1 = rowranges[jb]
              if ja < len(ranges2d):
                  _, arow1, acol0, acol1 = ranges2d[ja]
                  if arow1 < irow:
                      ja += 1
                      continue
                  assert arow1 == irow or arow1 == irow + 1
              else:
                  acol0 = acol1 = 1000000000
  
              if bcol0 == acol0 and bcol1 == acol1:
                  ranges2d[ja][1] = irow + 1
                  ja += 1
                  jb += 1
              elif bcol1 <= acol0:
                  ranges2d.insert(ja, [irow, irow + 1, bcol0, bcol1])
                  ja += 1
                  jb += 1
              elif bcol0 >= acol1:
                  ja += 1
              else:
                  assert ja < len(ranges2d)
                  ranges2d[ja][1] = irow + 1
                  if bcol0 < acol0:
                      ranges2d[ja][2] = bcol0
                  if bcol1 > acol1:
                      ranges2d[ja][3] = acol1 = bcol1
                      ja = _collapse_ranges(ranges2d, ja)
                  jb += 1
      return ranges2d
docs: This function takes a list of row-ranges (as returned by `_parse_row`)
      ordered by rows, and produces a list of distinct rectangular ranges
      within this grid.
  
      Within this function we define a 2d-range as a rectangular set of cells
      such that:
        - there are no empty rows / columns within this rectangle;
        - the rectangle is surrounded by empty rows / columns on all sides;
        - no subset of this rectangle comprises a valid 2d-range;
        - separate 2d-ranges are allowed to touch at a corner.