XOR-trick and PUZZLE (was comma-operator)

[Miles Bader]

> How can you use the XOR trick to keep a doubly linked list, using only
> one field for in each record, to code for finding both the following and 
> the previous record?  

To make it a bit clearer, you have to be able to find the NEXT record
given the current and previous records, and find the PREVIOUS record
given the current and the next record.