The digital 'xnor' gate is an n-input, single-output 'xnor' gate which
produces an active "0" value if an odd number of its inputs are also "1"
values. It produces a "1" output when an even number of "1" values occours
on its inputs. 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. Note also that to maintain the technology-independence
of the model, any UNKNOWN input, or any floating input causes the output to
also go UNKNOWN.