text: module.exports = (vs) => {
    // Add an array of vectors together. See vector.add to add two vectors.
    //
    // Parameters
    //   vs
    //     array of vectors. The array can be empty.
    //
    // Return
    //   a vector
    //
    const n = vs.length
  
    let sumx = 0
    let sumy = 0
    for (let i = 0; i < n; i += 1) {
      sumx += vs[i].x
      sumy += vs[i].y
    }
  
    return {
      x: sumx,
      y: sumy
    }
  }
  