code: function range(start, stop, step){
    utils.assertType(start, 'unit', 'start');
    utils.assertType(stop, 'unit', 'stop');
    if (step) {
      utils.assertType(step, 'unit', 'step');
      if (0 == step.val) {
        throw new Error('ArgumentError: "step" argument must not be zero');
      }
    } else {
      step = new nodes.Unit(1);
    }
    var list = new nodes.Expression;
    for (var i = start.val; i <= stop.val; i += step.val) {
      list.push(new nodes.Unit(i, start.type));
    }
    return list;
  }
docs: Returns a list of units from `start` to `stop`
  by `step`. If `step` argument is omitted,
  it defaults to 1.
  
  @param {Unit} start
  @param {Unit} stop
  @param {Unit} [step]
  @return {Expression}
  @api public