errors which occur on 
   Let's go through all the double errors possibilities:
   data-data: CANNOT be corrected, 
	      because cannot be differentiated 
	      from a double error in two adjacent parity bits

   data-parity: CANNOT be corrected, 
		because 
		1. if the parity bit is related to the data bit,
		can be confused with an error on the other related,
		parity bit of the data bit;
		2. if the parity bit is unrelated to the data bit,
		can be confused with an error on another data bit
		(the one mapped by the erroneous parity bit and
		and one of the parity bit of the erroneous data bit)

   parity-parity: 
Double errors that occur in two parity bits of the same type
   (i.e. rows or columns) can be corrected because they lead
   to a contradiction if it assumed to be a one error.