A negation property is same as logic not operator on outcome of a propery. So if the outcome is true, then negation makes it false. Below example which is modified version of earlier example.

  1 //+++++++++++++++++++++++++++++++++++++++++++++++++
  2 //   DUT With assertions
  3 //+++++++++++++++++++++++++++++++++++++++++++++++++
  4 module negation_assertion();
  5 
  6 logic req,gnt,clk;
  7 //=================================================
  8 //  Clock generator
  9 //=================================================
 10 initial begin
 11  clk = 0; 
 12  forever  #1  clk ++;
 13 end
 14 //=================================================
 15 // Simple DUT behaviour
 16 //=================================================
 17 always @ (posedge clk)
 18   gnt <= req;
 19 //=================================================
 20 // Test Vector generation
 21 //=================================================
 22 initial begin
 23   req <= 0;
 24    #3  req <= 1;
 25    #5  req <= 0;
 26    #1  $finish;
 27 end
 28 //=================================================
 29 // A negation property
 30 //=================================================
 31 property negation_prop;
 32   @ (posedge clk)
 33     not (req  ##1  gnt);
 34 endproperty
 35 //=================================================
 36 // Assertion Directive Layer
 37 //=================================================
 38 negation_assert : assert property (negation_prop);
 39 
 40 endmodule

 "negation_assertion.sv", 38: 
 negation_assertion.negation_assert: started at 5s failed at 7s

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