code: function make(unitDegrees) {
    Uu.compact(unitDegrees);
    unitDegrees.sort(sortByDegreeThenAbbrev);
  
    let sid = '';
    for (const ud of unitDegrees) {
      sid += ud.unit.sid;
      sid += ud.degree;
    }
  
    let units = bySid[sid];
  
    if (!units) {
      units = new Units(sid, unitDegrees);
      bySid[sid] = units;
    }
  
    return units;
  }
docs: Unit Parsing