//`default_nettype none

// top level module must be named "wokwi"
 
// interesting stuff is printed if the name is different
 

module wokwi ( input  CLK, output LED0, output LED1, output LED2, output LED3 );

    assign LED0 = CLK;            // this works

    wire [3:0] xyz;          // this is required
    assign LED1 = xyz[1];    // this is required

    blink abc123( .clk  (CLK), .led0 (LED0), .led1 (xyz[1]), .led2 (LED2), .led3 (LED3) );

endmodule

module blink ( input wire clk, output wire led1, output wire led2 );

    wire [1:0] counter;
//  assign counter = 2'b11;
    assign counter[0] = clk;

    assign led1 = counter[0];

    assign led2 = counter;        // this equates to bit 0 of <counter>
                                  // when <wire [1:0] counter;> is not declared

endmodule