code: function clone(dur, alts, clear = false) {
    // deep merge for vals
    const conf = {
      values: clear ? alts.values : Object.assign({}, dur.values, alts.values || {}),
      loc: dur.loc.clone(alts.loc),
      conversionAccuracy: alts.conversionAccuracy || dur.conversionAccuracy
    };
    return new Duration(conf);
  }
docs: clone really means "create another instance just like this one, but with these changes"