function mousedown(e) {
let coords = screenToSVGCoords(canvas, e);
let initial_coords = mutable rect_coords;
mutable delta = {
x: coords.x - initial_coords.x,
y: coords.y - initial_coords.y
};
document.addEventListener('mousemove', mousemove);
document.addEventListener('mouseup', mouseup);
rect.classList.add('moving');
e.preventDefault();
}