module dff1 (D, CLK, Q);
  reg IQ, IQN;
  input D;
  input CLK;
  output Q;
  assign Q = IQ; // IQ
  always @(posedge CLK) begin
      // !D
      IQ <= (~D);
      IQN <= ~((~D));
  end
endmodule
module dff2 (D, CLK, Q);
  reg IQ, IQN;
  input D;
  input CLK;
  output Q;
  assign Q = IQ; // "IQ"
  always @(posedge CLK) begin
      // D '
      IQ <= (~D);
      IQN <= ~((~D));
  end
endmodule
module dffe (D, EN, CLK, Q, QN);
  reg IQ, IQN;
  input D;
  input EN;
  input CLK;
  output Q;
  assign Q = IQ; // "IQ"
  output QN;
  assign QN = IQN; // "IQN"
  always @(negedge CLK) begin
      //  ( D & EN ) | ( IQ & ! EN )
      IQ <= ((D&EN)|(IQ&(~EN)));
      IQN <= ~(((D&EN)|(IQ&(~EN))));
  end
endmodule