The digital 'xor' gate is an n-input, single-output 'xor' gate which
produces an active "1" value if an odd number of its inputs are also "1"
values. 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.