function invert$4(out, a) {
    var aa = a[0],
        ab = a[1],
        ac = a[2],
        ad = a[3];
    var atx = a[4],
        aty = a[5];
    var det = aa * ad - ab * ac;

    if (!det) {
      return null;
    }

    det = 1.0 / det;
    out[0] = ad * det;
    out[1] = -ab * det;
    out[2] = -ac * det;
    out[3] = aa * det;
    out[4] = (ac * aty - ad * atx) * det;
    out[5] = (ab * atx - aa * aty) * det;
    return out;
  }
Inverts a mat2d

@param {mat2d} out the receiving matrix
@param {ReadonlyMat2d} a the source matrix
@returns {mat2d} out