code: def _arg_varname(self, wire):
          
          if isinstance(wire, (Input, Register)):
              return 'd[' + repr(wire.name) + ']'  # passed in
          elif isinstance(wire, Const):
              return str(wire.val)  # hardcoded
          else:
              return self._varname(wire)
docs: Input, Const, and Registers have special input values