algebra.index(set, items, item)

Returns where item should be inserted into items which is represented by set.

algebra = new set.Algebra(
  set.props.sort("orderBy")
);
algebra.index(
  {orderBy: "age"},
  [{id: 1, age: 3},{id: 2, age: 5},{id: 3, age: 8},{id: 4, age: 10}],
  {id: 6, age: 3}
)  //-> 2

The default sort property is what is specified by id. This means if that if the sort property is not specified, it will assume the set is sorted by the specified id property.