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For set A B C, you can enumerate the powerset in binary:
...
0001   D
0000

(for ABC)
...
Have we seen C before? YES - return 2
Have we seen D before? No? D == slot 3

By using a dead bit for A, we actually have this:
...
0001   D
0000

11111111111111111111111111  invalid
...
00000000000000000000000010  invalid
00000000000000000000000001  valid (z)

From up above, set (z)
...
0001    D    (0001 & 0010 == 0)
0000 ()        (0000 & 0010 == 0)

use strict;
...

print "TOTAL CALLS $calls\n";

#!/usr/bin/perl
...
    @skippers{@future_skippers} = (1) x @future_skippers;
}