SPUG: How can I find @1 - @2?

[Yitzchak Scott-Thoennes]

> * Ken Clarke <kenslinux at shaw.ca>
>> my %enrolled = map { $_ => 1} @enrolled;
>
> # No map nor 1's floating around in memory.
> my %enrolled; @enrolled{@enrolled} = ();

And changing $enrolled{...} to exists $enrolled{...}