/usr/share/tkgate/primitives/newdemux.v is in tkgate-data 2.0~b10-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16  | module newdemux #(.Dez(1), .Diz(1)) (I, S, $${NUMOUT:, :Z%});
   input ${I_RANGE} I;
   input ${S_RANGE} S;
   output ${I_RANGE} $${NUMOUT:, :Z%};
   wire ${OUT_RANGE} Zcat;
   specify
$${NUMOUT:\n:      (S *> Z%) = Dsz; } 
$${NUMOUT:\n:      (I *> Z%) = Diz; } 
   endspecify
   assign Zcat = ${invZ} ( ((${NUMOUT}'b1 << I) != 0) ? I : ${I_BITS}'h0;
$${NUMOUT:\n:   assign Z% = Zcat[%];} 
   
endmodule // demux
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