It computes an order isomorphism between two enumerated sets of rational numbers.

There are four predefined enumerated sets:

List 0 is an enumeration of all the rational numbers

List 1 is an enumeration of the rational numbers a/b in (0,1) arranged by a+b

List 2 is an enumeration of the rational numbers a/b in (0,1) arranged by b

List 3 is an enumeration of the dyadic rational numbers in (0,1)