List<Signal> signals: This private member acts as a repository for all the signals which have been transmitted through the wire during the course of the simulation. It is simply a linked list of Signal objects.
void display_signals(): This helper method employed by show_signals() iterates over the signal list and displays all the signal values and times currently on the wire. Only the changes in signal values are actually displayed by this function.
void replace(Signal sig): This function is used to replace a signal in the list with the specified signal. It is intended for use during the simulation of zero-delay elements but is not used by the current implementation.