text: const sum = (x, y) => x + y;
  const square = (x) => x * x;
  
  const mean = (data) => data.reduce(sum) / data.length;
  const stddev = function (d) {
    let m = mean(d);
    return Math.sqrt(
      d
        .map((x) => x - m)
        .map(square)
        .reduce(sum) /
        (d.length - 1)
    );
  };
  
  module.exports = { mean, stddev }; // default
  