[documentation] How to use Flattening?

[hxse]

https://codesandbox.io/s/perfect-freehand-example-forked-szhrx6?file=/src/App.js

const pathData2 = getSvgPathFromStroke(stroke);
const pathData = getFlatSvgPathFromStroke(stroke);

I want to draw smooth lines, without intersect points.
But i get error Input geometry is not a valid Polygon or MultiPolygon