The digital 'nand' gate is an n-input, single-output 'nand' gate which
produces an active "0" value if, and only if, all of its inputs are also "1"
values. If any of the inputs is a "0"; the output will also he a "1"; 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.