module dff(clk, d, q);
input clk, d;
output reg q;
always @(posedge clk)
	q <= d;
endmodule

module dffa(clk, arst, d, q);
input clk, arst, d;
output reg q;
always @(posedge clk or posedge arst) begin
	if (arst)
		q <= 1;
	else
		q <= d;
end
endmodule

module dffa1(clk, arst, d, q);
input clk, arst, d;
output reg q;
always @(posedge clk or negedge arst) begin
	if (~arst)
		q <= 0;
	else
		q <= d;
end
endmodule

module dffa2(clk, arst, d, q);
input clk, arst, d;
output reg q;
always @(posedge clk or negedge arst) begin
	if (!arst)
		q <= 0;
	else
		q <= d;
end
endmodule

module dffa3(clk, arst, d, q);
input clk, arst, d;
output reg q;
always @(posedge clk or negedge arst) begin
	if (~(!arst))
		q <= d;
	else
		q <= 1;
end
endmodule

module dffa4(clk, arst1, arst2, arst3, d, q);
input clk, arst1, arst2, arst3, d;
output reg q;
always @(posedge clk, posedge arst1, posedge arst2, negedge arst3) begin
	if (arst1)
		q <= 0;
	else if (arst2)
		q <= 0;
	else if (!arst3)
		q <= 0;
	else
		q <= d;
end
endmodule

module dffsr1(clk, arst, d, q);
input clk, arst, d;
output reg q;
always @(posedge clk, posedge arst) begin
	if (arst)
		q <= d^d; // constant expression -- but the frontend optimizer does not know that..
	else
		q <= d;
end
endmodule

// module dffsr2(clk, preset, clear, d, q);
// input clk, preset, clear, d;
// output reg q;
// always @(posedge clk, posedge preset, posedge clear) begin
// 	if (preset)
// 		q <= 1;
// 	else if (clear)
// 		q <= 0;
// 	else
// 		q <= d;
// end
// endmodule