The inverter is a single-input, single-output digital inverter which
produces as output an inverted, time-delayed copy of its input. The delays
associated with an output rise and those associated with an output fall may
be specified independently. The model also posts an input load value (in
farads) based on the parameter input_load. The output of this model does
NOT, however, respond to the total loading it sees on its output; it will
always drive the output strongly with the specified delays.