<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _arg_varname(self, wire):
 """ Input, Const, and Registers have special input values """

        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)