function setup() {
  createCanvas(200, 200);
}

function draw() {
  background(0);
  stroke(255);
  strokeWeight(10);
  line(50, 50, 50, 150);
  line(50, 150, 150, 150);
  line(150, 150, 150, 50);
  line(150, 50, 50, 50);
}