the addressing system of which is partially looped through the intermediary of a third set of buffer registers 46 the read only memory 45 stores m sequences made up of p data items that can each take m distinct values and which correspond to the m specific arrangements of p pattern types defining the recurrent composition laws each data item in a sequence identifying a pattern type occuring at the same rank in the corresponding arrangement it has two addressing inputs a row addressing input 47 for selecting the m sequences one by one and a column addressing input 48 for selecting the data items appearing at a particular rank in the sequences the column addressing input 48 is controlled by the data output of the multiplexer 41 and the row addressing input 47 is addressed by a data item read previously and stored temporarily in the third set of buffer registers 46 the third set of buffer registers 46 connected between the row addressing input 48 of the memory 45 and its data read output 49 is provided with presetting control means 39 connected to the reset to zero input of the address counter 42 and writing control means 38 connected to the output of the gate 44 each pulse which increments the address counter 42 corresponds to the end of the determination of a segment of the routing in.