Function sum

Adds both start and end points by given x,y

import { Lines } from 'https://unpkg.com/ixfx/dist/geometry.js'
// Line 1,1 -> 10,10
const l = Lines.fromNumbers(1,1,10,10);
const ll = Lines.sum(l, {x:2, y:4});
// Yields: 3,5 -> 12,14
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