The digital 'nor' gate is an n-input, single-output 'nor' gate which
produces an active "0" value if at least one of its inputs is a "1" value.
The gate produces a "1" value if all inputs are "0"; if neither of these
conditions holds, the output will be unknown. 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.