Pointers and recursive type definitions complicate the algorithm for determining structural equivalence of types. Consider, for example, the following definitions:
type A = record
    x : pointer to B
    y : real

type B = record
    x : pointer to A
    y : real

The simple definition of structural equivalence given in Section 7.2.1 (expand the subparts recursively until all you have is a string of built-in types and type constructors; then compare them) does not work: we get an infinite expansion (type A = record x : pointer to record x : pointer to record x : pointer to record . . . ). The obvious\ reinterpretation is to say two types A and B are equivalent if any sequence of field selections, array subscripts, pointer dereferences, and other operations that takes one down into the structure of A, and that ends at a built-in type, always encounters the same field names, and ends at the same built-in type when used to dive into the structure of B—and vice versa. Under this reinterpretation, A and B above have the same type. Give an algorithm based on this reinterpretation that could be used in a compiler to determine structural equivalence.