Ok, I understand all of this except for the 'x while.' I understand that x is a repetition operator but I've always seen it written like: '"a" x 80.' However here, there is nothing on the left hand side. Could you explain what it does? I'm guessing it repeats the substitution so that it can find the nested cases but I'd be interested in knowing how it works.
It's not the (lower-case) x operator, it's an (upper-case) unquoted string. Its purpose is to do nothing, but allow there to be a while loop. All the action of the while loop takes place inside its condition.
Caution: Contents may have been coded under pressure.