Simulation with verilog-a model

I am trying to simulate a memristor model and check basic operations.

I got memristor source code written with verilog-a(below).

module memristor (p, n) ;

inout p, n ;
electrical p, n ;

parameter real uv = 10f;
parameter real d = 10n;
parameter real ron = 100;
parameter real roff = 38k;
parameter real rin = 5k;

real k, r1, r2, R;

analog begin

k = 2 * uv * ron * (roff - ron) / pow(d,2);
r1 = pow(rin,2) + k * idt( V(p,n), 0 );
r2 = min( pow(roff,2) , max(r1,pow(ron,2) ) );
R = sqrt(r2);
V(p,n) <+ R * I(p,n) ;

end

endmodule

And I created a cellview to add sin wave voltage source like below.

But when I simulated above module with ADE XL, Result is 

Memristor has a constant resistance rin(of above code).

I think idt(...) of the code does not work. Should I do something more when simulate with verilog-a module? 

Thank you.