cornor thingy - OpenProcessing

mySketch
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let a, b;
​
function setup() {
  createCanvas(windowWidth, windowHeight);
  background(100);
  createCanvas(480, 480);
​
  background(255);
​
  stroke(0);
​
  a = 0;
  b=0;
​
}
​
function draw() {
  
  line(0, a, a, 480);
​
  if (a < 480) {
​
    a = a + 10;
​
  }
  
  line(500, b, b, 0);
​
  if (b < 480) {
​
    b = b + 10;
​
  }
  
}