net currently being processed in relation to the cell or input port output port etc being examined or just examined if all the cells on this net have been examined 30 and if one of the input pin s of the last cell examined is connected to the input port 32 this input port has been completely processed and the next untraced input port is searched as indicated at 22 if all the cells on the net have been examined 30 but none of the input pin s of the last cell just examined is connected to the input port 32 there are still cells between the cell just examined and the input port thus it is necessary to trace the net connected to the input pin of the last cell examined 34 if all the cells on this net have not been examined 30 and if the cell is a storage element 36 the cell is needed for the shell and is marked as kept 38 the shell generation process then returns to box 28 to examine the next unexamined cell connected to this net if per test 36 this cell is not a storage element the cell is kept as part of the shell and the net connected to the output pin of this cell is traced 40 if the output pin is connected to an output port 42 the cell is marked as kept 44 if the output pin is not connected to an output port 42 the shell generation process continues to find an unexamined cell connected to this net 28 this process recursively forward traces the net s.