1. A method of enabling the following up of the development of an error in a computation sequence during an operation in progress, in which each computation gives a partial result, and the partial results taken together give a final result, and in which there is included a series of transfers of binary information words between a plurality of processing units which from a received information word a new information word is formed for transfer to a subsequent processing unit, said method comprising adding a marking label to an information word which initiates a computation sequence to be investigated, in each processing unit to which said labelled word is subsequently supplied, removing the label and storing the same and then adding the label to the resulting information word formed in the associated processing unit, sensing each information word transferred between the processing units and copying the labelled information words transferred between the processing units into a memory, and displaying said information words which have been copied in said memory whereby to enable determining whether the words in the sequence of labelled information are correct.