code: function union (frame, otherFrame) {
    var x1 = Math.min(frame.x, otherFrame.x);
    var x2 = Math.max(frame.x + frame.width, otherFrame.x + otherFrame.width);
    var y1 = Math.min(frame.y, otherFrame.y);
    var y2 = Math.max(frame.y + frame.height, otherFrame.y + otherFrame.height);
    return make(x1, y1, x2 - x1, y2 - y1);
  }
docs: Compute the union of two frames
  
  @param {Frame} frame
  @param {Frame} otherFrame
  @return {Frame}