veriloga spectre error

Below is the veriloga code i wanted to run in spectre to record zero crossing times.

I am getting the error:

Error found by spectre during AHDL compile.
    ERROR (VACOMP-2259): "last=$last_crossing<<--?  (V(in), +1); "
        "/home/srm/cadencesch/varcap/last_cros/veriloga/veriloga.va", line 17:
        syntax error.

// VerilogA for varcap, last_cros, veriloga

`include "constants.vams"
`include "disciplines.vams"

module last_cros(in);
input in;
voltage in;
parameter integer dir=1 from [-1:1] exclude 0;
integer fp;
real last;

analog begin
last = 1.0;
@(initial_step)
fp= $fopen("zerocrossings");
last=$last_crossing (V(in), dir);             <--------------------------------------line number 17

@(cross(V(in), dir))
$fstrobe( fp, "%0.10e", last);

@(final_step)
$fclose(fp);

end
endmodule