func (s IndexStep) Apply(val Value) (Value, error) {
	if val == NilVal || val.IsNull() {
		return NilVal, errors.New("cannot index a null value")
	}

	switch s.Key.Type() {
	case Number:
		if !(val.Type().IsListType() || val.Type().IsTupleType()) {
			return NilVal, errors.New("not a list type")
		}
	case String:
		if !val.Type().IsMapType() {
			return NilVal, errors.New("not a map type")
		}
	default:
		return NilVal, errors.New("key value not number or string")
	}

	has := val.HasIndex(s.Key)
	if !has.IsKnown() {
		return UnknownVal(val.Type().ElementType()), nil
	}
	if !has.True() {
		return NilVal, errors.New("value does not have given index key")
	}

	return val.Index(s.Key), nil
}

type Set struct {
	vals  map[int][]interface{}
	rules Rules
}

// Rules represents the operations that define membership for a Set.
//
// Each Set has a Rules instance, whose methods must satisfy the interface
// contracts given below for any value that will be added to the set.
type Rules interface {
	// Hash returns an int that somewhat-uniquely identifies the given value.
	//
	// A good hash function will minimize collisions for values that will be
	// added to the set, though collisions *are* permitted. Collisions will
	// simply reduce the efficiency of operations on the set.
	Hash(interface{}) int

	// Equivalent returns true if and only if the two values are considered
	// equivalent for the sake of set membership. Two values that are
	// equivalent cannot exist in the set at the same time, and if two
	// equivalent values are added it is undefined which one will be
	// returned when enumerating all of the set members.
	//
	// Two values that are equivalent *must* result in the same hash value,
	// though it is *not* required that two values with the same hash value
	// be equivalent.
	Equivalent(interface{}, interface{}) bool
}