docs: def IndexToColumn (ndx):
          """convert index to column. Eg: IndexToColumn(2) = "B", IndexToColumn(28) = "AB" """
          ndx -= 1
          col  = chr(ndx % 26 + 65)
          while (ndx > 25):
                  ndx  = ndx // 26
                  col  = chr(ndx % 26 + 64) + col
          return col