Traceback (most recent call last):
  File "generate_netlist.py", line 192, in <module>
    create_chips(chip_types, nets)
  File "generate_netlist.py", line 123, in create_chips
    make_techmap(new_74151, mapping, instances, nets)
  File "generate_netlist.py", line 75, in make_techmap
    chip[ch.format(i+base)] += nets[a]
TypeError: unsupported operand type(s) for +=: 'NoneType' and 'Net'