code: function unit(unit, type){
    utils.assertType(unit, 'unit', 'unit');
  
    // Assign
    if (type) {
      utils.assertString(type, 'type');
      return new nodes.Unit(unit.val, type.string);
    } else {
      return unit.type || '';
    }
  }
docs: Assign `type` to the given `unit` or return `unit`'s type.
  
  @param {Unit} unit
  @param {String|Ident} type
  @return {Unit}
  @api public