Hi, I have been fiddling with a program very similar to EXAMPLES/DRAWING/PATTERN with minor differences.
 The main being that the draw() is executed while the LEFT mouseButton is held, and my Intention was that the other mouseButton 's would clear the screen.
 The problem is after the screen clears, the program Seems finished, and you can no longer draw. I'm pretty sure the code I have would work if was moved around a bit, but I've tried many variations ( nesting functions within draw(), functions called, function order changed) I'm stuck! The code in it's entirety:

//start
float circleSize;

void setup() {
 size(200, 200);
 background(255);
 stroke(0, 102, 153, 204);
 fill(0, 102, 153, 50);
 noLoop();
}

void draw() {
 delay(10);    //changes the 'feel' of the program
 circleSize = abs(mouseX - pmouseX);  //calculates absolute difference between two mouse positions
 circles(mouseX, mouseY);  //defines mouse position for circles() function
}

void circles(int x, int y) {
 ellipse(x, y, circleSize, circleSize);  //draws circles
}

void clearScreen() {
 noStroke();
 fill(255);
 rect(-10, -10, 210, 210);
 loop();
}

void mousePressed() {
 if (mouseButton == LEFT) {
 loop();
 } else {        //  any mouseButton except LEFT
   clearScreen();//    will call clearScreen
 }
}

void mouseReleased() {
 noLoop();
}
//end