cells_sim.v

module inv (
  output Q,
  input A
);
  assign Q = A ? 0 : 1;
endmodule

module buff (
  output Q,
  input A
);
  assign Q = A;
endmodule

module logic_0 (
  output a
);
  assign a = 0;
endmodule

module logic_1 (
  output a
);
  assign a = 1;
endmodule

(* blackbox *)
module gclkbuff (
  input A,
  output Z
);

  specify
    (A => Z) = 0;
  endspecify

  assign Z = A;

endmodule